Is there any way to tell when a user session starts, and when it ends? I am wanting to indicate to other users when users in their group are online.
A session starting trigger could be created with core development. If there is no existing session ID, we could fire a trigger event when the session starts, but this would be for logged in and not logged in users. What would be better for you would be to use the Login trigger event, but you wont know if they are a returning user because Matrix doesn't keep track of their last visit time.
So what I really think you need, and the way these sort of features work normally I'd say, is for Matrix to keep track of last activity and for you to be able to grab a list of users who are currently active (i.e., activity in the last x mins). There is no way of doing this at the moment and it would require core dev (+ DB schema change) to keep track of it.
Thanks Greg for the info on this.