Is is possible to add a user to an group message that is already in place? In slack if you @mention someone in a group message it invites them to the group message.
Thanks for your question!
You can add a new user to your group message by clicking on the “v” to the right of the group message name in the center channel. This will, however, create a new conversation and the updated group will appear in the left-hand-side panel under “Direct Messages”.
You can read about group messaging here
Does this help?