You will be removed from a group if you are not active in that particular group for 30 days. Performing the following actions qualifies as being active in the group:
Creating a game session
Joining a game session
Posting a message in the group chat room
If you are inactive for a period of time, you should receive an email asking, "Do you like [group name]?" If you wish to stay in the group, select the "I want to stay in this group" option, and that will reset your inactivity timer.
If you are removed from a group and can't find any emails like what is described above, you can search for your previous group and rejoin it using the "Join" button on the group page. Finally, if you don't remember which group you were previously a member of, or the group is full when you attempt to rejoin, you can email mike(at)the100.io and he can get you added back into your former group.