The posts with more than one person are posts that involve more than one person's characters.
If you have a post that only involves your character(s) then you just have to write it up and post it. But if you want to do a post with someone else's character as well just hit them up with a PM, a thread or even add them on MSN (I thought there was a contact thread around somewhere but I've lost it... Just ask, I'm sure the nice members won't say no :p).
That answer your question?
