Clans are there because people like to gather in groups.
You join usually by telling the leader you join, otherwise requirements will be listed.
There are only the reward of unity with the rest of the clan, as well as whatever perks the clan will give you.
After joining a clan, you are in a clan.
From a more AG related PoV:
Clans are usually made to gain a unity in a multiplayer game, such as Exit Path or any like those. Users will then usually make an alternate account to demonstrate their belonging in a certain clan, and will then play and win to get the popularity and fame of that clan to increase, thus getting popular and famous themselves. Other related words would be group or club, which are pretty much the same, but usually work outside of multiplayer games. It unites users with similar interests, usually for the reason of uniting people of similar interests.
These clans, groups or clubs often have little requirements when it comes to joining. Clans related to multiplayer games might ask of a certain highscore, or a good track record in that game, while clubs and groups might ask some questions to see if you are "good enough" to join in. Usually, however, all you have to do is ask.
There is no official reward in joining a clan. You do it for your own amusement, and that is it.
After you join a clan, you are perhaps required to play a certain amount a week, keep winning or something similar. This is usually explained when you join the clan, and if not, it is just as often detailed on the leader's profile or somewhere similarly easy to find. You should, however, not go about making a thread about the clan, advertise the clan to others by spamming their profiles asking them to join, or otherwise bend the rules for your clan. Because that is annoying. Also, only one alternate account per multiplayer game, and all that.
