Mobile:
Go to Messages tab
Select Message to add User(s)
Click on Group Message name at top of screen
Click Add Participants
Select User(s) to add to Group Message
Click Done in top right corner
Web:
Go to Messages tab
Select Message to add User(s)
Click 3 dots (⋮) in top right corner
Click See Conversation Information
Click Add Participants
Select User(s) to add to Group Message
Click Save