Thanks for that info and choosing the Apple Support Communities. If I understand correctly, you are unable to add a new co-worker to a group message thread in the Messages app on your iPhone. If you haven't already, please force close the Messages app and restart your iPhone. Then follow these steps to add this co-worker to this group message if they are also using an iPhone:
Add or remove someone from a group conversation
Anyone in a group iMessage can add or remove someone from the conversation. You can remove a person from a group iMessage that has at least three other people. You can’t add or remove people from group MMS messages or group SMS messages.
Add a person
- Tap the group conversation you want to add someone to.
- Tap , then tap Add Contact.
- Enter the contact information for the person you want to add. Then tap Done.