In MaskGun, you must be at least at Level 3 to be able to join or create a clan.
Once you are at Level 3, tap the Clan tab on the homescreen and join any suggested clans displayed on the screen. Player can even 'Search' or 'Create' his own clan according to preference.
The following image shows the screen with recommended clans that a player can join.
The following image shows the screen where player can create his own clan.