Team Size Icons ?!

OK guys, time to test your patience again!

When I'm in areas for coop-missions and the like, some characters are standing around with blue character icons above their heads and a number between 2 and 4 (or max party size). What is it ?

I assume it is people 'advertising' to team up and waiting for enough players to come along, but I can't find out how to do it. Most people I ask about their 'blue char icon' look blankly at me! I've always just asked around to find party members, but if I'm not using the "correct procedure" I'd like to get it right.

Thanks in advance...

Its shown over the heads of party leaders, its to show how many people are already in the party.
Its automatic when you invite someone to join you in a party.

Doh!!!
Thanks

oh and you can see youre own