If you are an admin to a group and you want to make another admin, follow the below steps:
  1. Go to your group
  2. Tap on the three dots from the right upper corner
  3. Tap Manager Members
  4. Tap on the three dots next to the member you want to make an admin
  5. Select Make admin
  6. An invite to become an admin will be sent to that member and they will have to accept to become an admin