That tribe is fully absorbed by the conquering tribe - making joining them / earning their gear impossible. Once you’ve conquered a tribe, you can’t go back and join that tribe. Each tribe has one rival, and you can’t join a tribe that’s currently your rival.

NOTE: Whenever you join a tribe, you’ll immediately gain a “rival” tribe.

Go to the tribe HQ, you’ll be given the option to join them. As long as you’re not currently in a tribe that’s rivals with the tribe you want to join, they will not attack you. Stumbling on a tribe’s territory will give you an idea of where the tribe HQ is located. To do that, you need to find the tribes on your own. It seems like you can only join two tribes at the start of the game, but you actually can join any of the tribes in any order you want. If you’re wondering how the tribal conflict system works, here’s what we’ve learned after 12+ hours of gameplay. The factions add another layer of customization, and you can easily miss tons of cool stuff if you rush through their quests too quickly. You don’t even have to defeat all five rival factions - and you can seriously join any of them, at any time, even if the game doesn’t get around to tell you about that. Tribal Warfare is your first task, and you can go about it in lots of different ways. There’s nothing stopping you from betraying your faction and even joining it again at a later date, and there are reasons you might want to do it! Joining certain factions will also give you boosts to your light and dark meters - on top of the cool unique armor set, and a unique weapon. There are actually six factions total, and if you don’t go exploring on your own, you’d never know about all the others you can join. From the very beginning, it seems like you’re stuck with choosing one of two factions.

Biomutant really doesn’t explain one of its most important concepts - how the big war scenario actually works.

Biomutant tribe locations