You can find the overview of the existing post groups in the menu tile "Admin Dashboard".
To add more members to a post group, select the "Members" tab in the group settings. With the button "Add members" in the upper right corner you can click on the checkboxes to add single or multiple members.
With the three dots menu on the right side or via the drop-down menu "Select action" authorizations for the individual members can be assigned. You can choose the following options: "Appoint as group admin", "Downgrade to member" or "Remove from group".