When a person leaves the organization or no longer needs access to the group, you remove them from the Members page.
Remove a person
In the members table, use the remove action on the relevant row. A confirmation is asked before you validate. The person then loses access to the group.
Revoke an invitation
A still-pending invitation can be revoked the same way, from the invitations list.
Safety: you can't remove the last admin of a group. Appoint another admin first if needed.