By now, there is no administration process to remove a user from a group. A user is only removed from (a) group(s) if you delete the user itself from the Domino Directory.
So you have to do it manually. This makes it impossible to delegate this task to users that do not have EDITOR access to the DD.
I have coded an enhancement to the adminp years ago, which worked for us. See: https://www.eknori.de/2007-12-15/delete-group-members-using-the-administration-process/
Although you can create your own administration processes, I would like to see the "remove user from group(s)" process as part of the core adminp.