it means you can plurk to that select group of people. So if you have a group of people you always go to the pub with you can plurk just the

cliques.

I've been thinking Twitter needs that e.g. could set up a Brum bloggers/Paradise Circus group

isn't that what hashtags do? means people in other places can see and get ideas.

needs all the reminders she can get for blogmeets and alike

I liken it to DMs with multiple recipients