Mobile:
Go to Messages tab
Select Message to add User(s)
Click on the group message name at top of your screen
Click Add Participants
Select User(s) to add to Group Message
Click Done in top right corner
Web:
1. Go to Messages tab
2. Select Message to add User(s)
3. Click 3 dots (⋮) in top right corner
4. Click See Conversation Information
5. Click Add Participants
6. Select User(s) to add to Group Message
7. Click Save