It's not all that common, at least not that I've noticed, but there are a handful of threads like that. Typically, they say something about just jumping in. Here are two examples:
Dead Horse Tavern and Inn
Svetopia
Also, if you check some of the Request forum for threads marked with the green "Group" tag, some of them are relatively new. Newer ones typically have only a handful of posts. They can be moved down kind of quickly, though, so it might be best to check the second page as well. :3
As far as I know, it's generally not looked down upon to ask for a summary if there are an excessive amount of posts. :P If there's only around twenty or so, I personally think it's better to read them.