Create new “User Groups”

Hi – any ideas on this one – it seemed a pretty obvious thing for people to make into a plug-in to me, but I’ve been looking for days and can’t find one…

I want to be able to put different users into different categories – but not change anyone’s access rights.

For example – say I have Bob, John, Sue & Alison on my blog – I would like to create a group that says Bob & ALison play the guitar and John & Sue play keyboard… etc – you get the idea.

I would then like to be able to send an email out to, e.g., only the guitarists, or only the keyboard players etc.

I’d also like to be able to view lists of who is in which group.

Is this possible? Is there a plug in in existence that does this already?

Thanks

Paul C