To me, welcome
thread only means "just send me a congrats card, I'll get to it when I can... no theme included". Avatar party has a theme with picture postings. Whether they are there for it or not.
To some degree, I don't actually care if they "show up" or not... I like it when they do... but they don't HAVE to show up. I'd just do the avatar party at the scheduled time with the theme, and if they can show up, great, if not oh-well. Then, it can be either "synchronous" or "asynchronous". It is a forum after all, not a chat room. They can always page back. In other words... we pretend they are there...whether they are there or not. Everything is on schedule, and known in advance.
It's better for them, and more fun for us, if they can schedule it for when they can show up. But that isn't a requirement at all. The whole purpose of a forum is to let people post at their own speed and timing.