Thanks for that info and choosing the Apple Support Communities. If I understand correctly, you are unable to re-join a group iMessage on your iPhone. If you haven't already, please force close the Messages app and restart your device as iOS clears cached data and does other system optimizations when you turn your device off and on. Test your issue again, and then follow these steps from this Apple resource on group messages issues:
If you have issues with a group message
If you’re in a group message and stop receiving messages, check if you left the conversation. Open Messages and tap the group message. If you see the message You left the conversation, then either you left the conversation or you were removed from the group message. You can rejoin a group message only if someone in the group adds you.
You can also start a new group message. Here’s how:
- Open Messages and tap .
- Enter the phone numbers or email addresses of your contacts.
- Write a new message, then tap to send.
If you have other issues with a group message, you might need to delete the conversation and start a new one. To delete a group message:
- Open Messages and find the conversation that you want to delete.
- Swipe left over the conversation.
- Tap Delete.
When you delete a message, you can't recover it. If you want to save parts of a message, take a screenshot. To save an attachment in a message, touch and hold the attachment, tap More, then tap Save.
If you can't send or receive messages on your iPhone, iPad, or iPod touch - Apple Support
Force an app to close on your iPhone, iPad, or iPod touch
Restart your iPhone, iPad, or iPod touch - Apple Support
Cheers!