They can't still be a guard if you want them back in the party. If you've dismissed them from being a guard, on the overview page, there should be a "edit party" button at the bottom.
Go to sleep while wearing the succubus ring. You'll end up in the dream realm with Selene. Leave the location as if you were traveling on the real world. You'll see an extra place you can go to, an obelisk, go touch it and it will let you relive scenes.
