If this idea IS implemented do you know how much spam and farming that could create? Just sending it to every freakin' user on the site to farm. Ya know what i mean? =/
Spam, maybe, but comments wouldn't award points when the point system is changed, which will probably happen before the social interaction stuff is changed.
Group messages could be restricted to those who are invited and accepted to the group, so unless you message the owner to send a message to an entire group you aren't in, spam wouldn't really be a problem (besides, it could be deleted and/or reported easily).
Or only group leaders could send group messages... I doubt people would join spam groups, if they even could.