Make sure the group messaging option is enabled in the Messaging Settings. To verify this go to Messages > More > Settings > More Settings > Multimedia messages > Group conversations. Make sure the group conversations option is turned on.
If this is turned on and you still have the same issue you can try using a 3rd party messaging app. Two well-known and and reliable apps are Textra SMS and Google Messenger.
These apps basically replace your built-in messaging app and provide more robust features in terms of multimedia messages and group messages. Neither of these apps are owned, operated or maintained by AT&T so we cannot guarantee any level of success.
Hope this helps!
Rachel A AT&T Social Media Manager
AT&T Customer Care
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