A tribe is best compared to guilds in any MMORPG. However, unlike most guilds in games, a tribe works a little bit different. It's not all about having more and stronger members all grouping up at the same time, where the weaker members feel useless because they can't deal enough damage or any at all, and just end up dying over and over. But in a tribe in ARK, everyone can be usefull.
Quickly explained, a tribe is a group of people that work together on building and maintaining a base, supporting eachother, and go on raids together. A player could play alone without any problems, but there is a limit to how far a player can level up and unlock recipes. Within a tribe, you can spend your Engrams (skill points, so to say) on buildings, while someone else looks after weapons, saddles, crafting stations, and so on. You all work together and you all have to pitch in on common goals.
Having a specific role may feel like you're limiting the game for yourself, but that is not how you should see it. Instead, think of it like this: I would have lost more time doing this if I had to do everything myself. Besides, just because you're not a tamer, doesn't mean you can't collect the ingredients for narcotics. And even so, in a world inhabited by wild dinosaurs, you always have to watch your toes. You can't say that chopping trees is the most boring job in the world, when a pack of Dilophosaurusses could spit all over you at any moment.
Even so, you will probably not be limited to one job. While you'll never have enough Engrams to unlock everything, you do get quite a lot over the levels. If your job is to craft armor, then you'll probably have a lot of Engram points left to invest. So, what should your second job be? In order to be as productive as possible, you better look at which attributes you've upgraded. For example, someone who took the role of gathering resources, has probably invested alot in the Weight attribute. This person could serve as a packmule when raiding a base, or looting the corpses of his own tribe members before enemies are able to.
Now, let's take a look at what roles you might have in a tribe.
Basic role, one that everyone will probably have when starting anew. Like the name says, a gatherer gathers stuff. This mainly includes collecting building resources and food, but also for when a lootcrate has dropped nearby. In this role, you mainly focus on upgrading your Weight attribute, though extra points in stamina will help in getting more things done in the same amount of time. Engram points are best spent on tools, survival tools (such as the water jar) and storages, and could also find use in anything that could provide transportation, such as saddles and the raft.
These players have the task to look around, keeping an eye out for predators approaching your base, as well as (possible) hostile players. They usually ride on flying creatures to get the best view. Because of that, they are also usefull for quickly looting lootcrates. Since the most optimal way to scout is by flight, you don't need points in movement speed, though it might help when you are unable to fly, same for stamina. On a PVP server, it's recommended to invest in health points, or you could end up dead before falling to your death. Spend Engram points on saddles, armor, and ranged tools, so you are able to protect yourself. Don't forget to unlock the parachute and other nifty things, such as the spyglass and night vision goggles.
The players who control the gardens, providing a food (and water) source exclusively for the tribe members and dinosaurs. The world is for everyone, and when everyone is gathering berries from nature, there will eventually be nothing left, which is why it's good to have a farm and people who maintain them. Your stats don't really matter for this job. You could increase movement speed if your base is huge, though it will only save you a few seconds to get your food from A to B, so save your points for another role. Engram points on anything food and water related, such as the farming plots, fridge, water reservoir, and so on. Pipes are also good to invest in, though are better handled by the architect. Feeding trough for the dinos are good too, but are better for the tamers.
As the name suggests, this guy will focus on building your base and making it a safe haven. He also stands in for the repairations of the base. This job is more of a side job in PVE, as bases are most likely going to be damaged by other players, and in that case, split the Engram decisions. Points could be spend on weight, though if you don't mind making a lot of trips, you could save some points for another attribute. All engrams on building structures and water and electricity provisions.
Also straightforward: The guy who tames the dinosaurs for the tribe. Attributes are best spent on health and stamina, to survive attacks from dinosaurs retaliating, and being able to chase them down long enough should they try to make a run for it. Engram points on the feeding trough, armor and tools for taming (such as the bola, tranquilizer arrows and blunt weapons). A tamer could also spent points on saddles, though look up which saddles can be crafted and which ones can't. It would be a shame if you spent 80 Engram points on a Quetz platform saddle, when someone suddenly finds a blueprint for it.
The people that protect the tribe. Basically a role everyone can assume. Those who speciallize in this will spent points on health and stamina. Melee damage would be nice, but most battles will best be fought with a ranged weapon. Leave melee damage to the right dinosaurs, you just focus on staying alive. Engram points on weapons, ammo and armor, as well as offensive mechanisms, such as turrets. and catapults. Of course, just because you're not a warrior, doesn't mean you should hide in your base naked. Everyone has a right to carry a weapon, wear the best armor they can find and help to protect the base or raid another tribe. Just don't go running in headfirst, leave that to the actual warriors.
These are some of the most common roles you could have in a tribe. Of course, tribes can come up with different roles, where the farmer has to create his own farm, thus spending points to get his greenhouse structures. But keep in mind, you will most likely not be limited to one job. Chances are that you will probably take on one job, and when the tribe needs more work to be done somewhere else, you could step in, invest your Engram points in the required technologies, and help your tribe out there. Also, there is a way of resetting all your points, so if you're really getting tired of your roles, you could try to reset them. Just make sure that your tribe approves of your decision. It's no use losing a gatherer to get another tamer, while there's already enough dinosaurs that can't be housed.
So what do you do if you get tired of your job and you can't do something else? Well, find a new server, a new tribe and find a tribe that would want someone who would like you to the work you wanted to do. This way, you can take a break and meet new people. Just make sure you check back regularly on your other server. Nobody wants tribe members if they're never online.