Something I’ve been waiting for, you can now add, remove and promote members to owners directly from the mobile app on iOS and Android.
Many modern workers are mobile first if not mobile only, particularly first line workers which are a focus for Microsoft Teams.
Simply tap the three dots by a team name and click Manage Members to see members and owners and add members. Click on an existing member to promote them to owner or remove them from the team.
Reference What’s New in Microsoft Teams | May 2019