You can check the status by clicking on the Administrator tab. There are 2 columns: Pending & Connected. If they have a "0" in Pending and a number in Connected, then they have accepted their invites. If they have a number in Pending and a "0" in Connected, they have not accepted yet. You can send them a reminder by selecting them and clicking on "Message Administrators>Reminder".

Click on the Edit icon on the far right to see which group they can access. 

If there is an exclamation mark in the "To Be Invited", then this admin has not been invited yet. Please select them and click on Message Administrators>Invite Group Admins and  follow the prompts. 

If you are a group admin and want to know if any other group admins have accepted for your group, select the Administrator tab to see who is pending or connected.