Hello and welcome to Apple Support Communities LucyKnox2,
We see that you're having an issue when trying to add contacts to a group message. We'd be happy to provide you with some information that can help.
First, restart your iPhone and test to see if the issue remains.
Restart your iPhone
If the issue remains, the articles below can help.
Add and remove people in group text messages
Send a group text message on your iPhone, iPad, or iPod touch
If you're still unable to add contacts, respond back with some more details. Is this only happening with a specific contact, or any contact that you try to add? Which version of software are you using? Have you been able to create and add contacts in the past without an issue?
Thank you for using Apple Support Communities.