Group Text message does not send to all included or the name is red

ok, so I started a group text with 3 people. Two who I normally talk with everyday normally, seperately and as a group, but when I add a person this person's name comes up red.

I will do this as user a, user b and user c so you can keep track. 🙂

Send text to user a and b = fine we all talk

send text to user c = we can talk

tried to add user c to an existing group convo between a and b and user c's name comes up red and the "done" option does not allow me to add him.

fine, I back up and create a new message and send it however no user says they got anything. grrrrr

I have shut off mms, reset network settings, and rebooted. that did nothing. No one is blocking group messages so I dont know what else to look for here.

Any help or advice would be much appreciated

paul