I'm having this same issue with T-Mobile. I called them and verified everything is configured properly (provisioned), and everything is fine on their end. I've enabled Group Messaging in the options and still cant send or receive group messages. Texting to a single person however works fine.
In the Messaging page, tap the "+" button, this will create a new msg page.
On the top right, just tap the "+" to add the people u wanted to send msg to.
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