I think the size of the interface gives you some indication of how important the communication is. If it was really hard to send an email, we'd assume emails that were sent were the more important ones. By "size of the interface" I also mean how hard it is to use and understand.
So Glancing should be (firstly) as small as possible, so it doesn't appear important, and (secondly) actually glancing at someone should be the same as looking to see who's online, and no second step.
It should be...
...in order to make the effort and cognitive interruption of using the application in proportion to the magnitude of the transaction. Most of the functionality exists outside the application. People join the group by using the interfaces of other systems; they are pushed out to other applications for any other type of interaction. There's a level of implicature, a group model in the interaction logic, which determines what other applications are available as jump-out points.