Chris Russell shared new rules for messaging other members of LinkedIn Groups on this blog post: Linkedin Sets New Limits on Messaging Group Members
Basically, you now are limited to messaging only 15 times to other members of all (not each) LinkedIn Groups you are a member of.
I’ve never messaged more than 15 people in a month, but I know job seekers who are very good at reaching out to people… and this hampers their networking. Read more at the link above.