You can convert a group direct message into a team, especially when they start becoming standing discussions. 

1. Open the group direct message conversation you wish to convert to a team.
2. Tap the group name at the top of the screen.
3. Tap the Gear button at the upper-right corner of the screen, and then tap Convert to Team.
4. Tap Continue to confirm.
5. Once a group message is converted to a team, you can give it a new name.
  • You can also change the Team Type to Private or Public.
6. Tap Done or the check-mark icon to finish.

If you are the one who converted the group to a team, you will become the Team Admin.