Community admins are able to assign an admin or a moderator to a specific groups. To do so, follow the steps below:
Go to the group's page> click on the (...) button on the top-right next to the moderation button> click on Group Members> Click on the role in front of each member to change it from member to admin or moderator.
This only changes the role of that user inside the group. Meaning, if you change the role of a group member from member to admin, it only makes the user an admin inside that group, not the admin for the whole community.