Why not add a changable status in the friend list? You could mark yourself "away" so your friends won't think you're ignoring them as they repeatedly go "hey... HEY??? ARE YOU THERE???"

Another could be "invisible" so if you want to be a ninja and venture on your own you won't be bothered by anyone. Or "private" (or "do not disturb") where they see you're online, but messages and invites won't go through.