If you have voluntarily taken yourself out of a chat, you can rejoin that chat using the procedure below. If you were removed from the chat by someone else, you will need to be re-invited.
(Some personal info has been blurred out for privacy reasons)
You can also find similar content on the GroupMe website
From the Chat List
From the chat list screen, you need to bring up your settings.
Click the three horizontal bars in the top left corner
From the Settings Screen
From the settings screen, you need to bring up your profile. And then click the “Archive” button.
and then the pop-up to confirm.