Sounds like you want to look into our Presence feature.
Presence is designed to let users know when their friends are online. You can think of it like the old Xbox Live friends feature - where you can see which of your friends are online - and optionally what they are doing.
And Presence uses RTT - so users will be notified in realtime when the presence information of a followed user changes.
Presence works with Friends, Groups, and arbitrary lists of users.
More details here - http://getbraincloud.com/apidocs/apiref/#capi-presence
Hope that helps!