To invite admins to your group, you would be required to subscribe to a paid plan that allows for more than one admin.
Once you have subscribed:
Go to the group page
Tap the 👤 (person) icon on the top right corner of the screen.
Person icon on group home screen
Tap on the + ADD button to add a team member
Add button for adding Team members
Enter the email address of the team member you want to add to the group.
Popup to enter email id for adding new admin
Once you do so, your team member will receive an email with instructions to sign up.
After they sign up using the same email address, they will automatically be added as a team member of the group.