Unfortunately, I haven't seen an API call that does exactly what you want.

I have seen a number of addons that use a common channel to communicate and update automatically.

The idea is for all group members to use the same channel using a unique name entered by the jamba user. On logon, the jamba master would announce itself to the channel and ask for all online toons in the channel to respond. Each client would also announce login/logout to the channel, and the master would ping frequently (every minute?) to check for clients who logged off without notification.

The most recent code I saw to do this was for Guild Event Manager.

It's going to require quite a bit of manual coding unfortunately, so I'm not sure how worthwhile the time invested would be.