Hi. OK, I admit it; I'm behind on how all this stuff works. Here's the situation: I just bought a new MacBook Pro and migrated from the old one. I now have a friend who is visiting from afar and is using my old MacBook while she is here. If she tries to log-in to something like Facebook, then I am logged out of Facebook on my new Mac - that sort of thing. Can I "deactivate" the Mac she's using without causing me problems with my new Mac? Can I "reactivate" it once she's gone and update what I've done so that the old Mac is current with my new one? Am I making any sense? Thanks so much!