If the user is a member of an opt-out group but does not have read permission, the group is not listed under "My memberships", which makes it difficult for the member to leave the group. The user must navigate through the grouper tree to find the group, then choose "Leave group" from the More actions menu. The group IS displayed on the "Groups I can Join" tab with the "Join group" button. I would expect it to show up on the My memberships page with a Leave group button.
Oddly enough, the pagination/result count on the My memberships page includes the opt-out group, it just isn't displayed in the list of results.