What are the … for in the right corner of the screen?

The three dots in the upper right corner of a message group allow you to view additional details about the group.  Group Managers are able to see who the Managers and Members are.  A Member of the group will not see that information.  This is also where you can choose Clear Chat History to delete all chat records of the group.  If you have been added to the group by mistake or no longer need to be part of the group, tap Remove Myself from Group.

Managers that were given edit permission will see an Edit option in the upper right corner of the screen.  They can edit the group name, settings, or delete the message group.  Please note that deleting the group will remove it for everyone.