You can always add more people to the group chat - this includes teammates. However, you can also add members of the same club, or other Mingle users - as long as you follow them!
Go to the 'Messages' tab and select the group chat you wish to add a member to.
Click on the top bar to view the 'group info' page. Here you can see the current members of the chat.
Click on "add group members" to start adding group members. You can click on your team name to automatically select everyone in the team, or click the dropdown menu to expand the team and select users individually. To add members from outside your team, use the search bar at the top to add specific people. Please note that you need to be following the user you want to add to the group chat.
Confirm you want to add these members by pressing the blue 'tick' button in the bottom right of your screen.
The users you've selected have now been added to the group chat!
Please note that new members cannot see the history of the group chat when they enter.