MSN Groups or Yahoo Groups?

So I need to start a “group” much like those found on MSN or Yahoo for my job. This site will be accessed by teenagers and will contain information about activities, service opportunities, etc. (ok, ok it’s for the Girl Scouts). A little file sharing would be nice (pictures and such) but not really mecessary. I mostly need a teen-friendly site with info and a message board. So I’m looking for a secure (i.e. no pedophile) and free (this being the operative word) place to host a group.

I’m pretty familiar with MSN Groups, less so with Yahoo. From experience, which is the better site for groups? Is there another option I’m unaware of? Any advice or suggestions are much appreciated.

Thanks a (teeming) million

-Daisy