iOS differentiates messaging threads based off the specific iMessage ID used to address each recipient. Since iMessage can use both phone numbers and email addresses as iMessage IDs, this can cause multiple threads and much confusion.

To ensure that a conversation stays in the same thread, you must ensure you are using the same iMessage ID everywhere. To check which iMessage ID is being used for a contact in a particular thread:

1. Open the Messages app from your homescreen
2. Tap the thread
3. Tap the "Details" button in the top-right corner
4. Tap the "i" button in the far-right of a contact's row
5. The current iMessage ID will be highlighted in blue (it can be a phone number or email address)

 

Once you know the iMessage IDs for a thread you want to use and wish to ensure that you are using the same iMessage IDs for each contact in Group Text:

1. Open Group Text+ from your homescreen
2. Tap the "Edit" button in the top-right corner
3. Tap a contact or group of contacts
4. You will see the current iMessage ID listed underneath each contact's name
5. To change one, tap the red ⊖ to the left of the contact, then "Remove"
6. Tap "Add Contacts"
7. Begin typing the same contact's name
8. Select the row with the correct iMessage ID displayed underneath the contact's name