Find which session friend is in.

[admin@newphase.com]

>>>
Hover your mouse pointer over your friends name.
A popup will come up. This will tell you, amongst a lot of other info, if he/she is in a session or not.
If so, and if it's an 'open' session, a yellow [Click to join] link will be displayed. Click that.


Edit: just noticed it doesn't behave like this for all of my contacts(?)

Thanks Dimitri.  I will give that idea a try.