Why can't i send group texts with iphone 4s?

I've had my new iphone 4s for a few days and love it, but it won't let me send texts to more than one person at a time. I can send individual texts just fine. It will let me add the recipients and compose it but when i send, the send progress bar gets almost to the end and then stalls for about 1 min, then i get the red exclamation point and the option to retry which i do but it never works. the group message option is set to ON but still it doesn't work. Anyone else having this problem?