I'd also suggest 2 things on this:
- a ticking option on the login screen that allows 'log in anonymously': it's just like when you're actually online but setting your online status to 'offline'
- if your login status is now 'offline', or the 'anonymous login option' is ticked, you won't receive any system notifications when player in your friend list has just logged in. This helps avoid the too laggy situation when friend keep relogging.