Dragana Markovic's dashboard

I believe you can't. They do this to prevent spamming (i.e. honest persons from making the living). I believe there is no other way to overcome this but to send individual messages to each and every member of your group.

I bought a domain name and hosting and created site similar to facebook groups and then invited people to join my own group on my own server and it worked wonders. It's easy to message 50.000+ members.

Hope this helps!