Any current "Captain," can upgrade any other group member to the role of "Captain" at any time. Here's how:
Go to your Admin > Members page.
Find the person to whom you want to grant admin privileges, and click the small blue "up" arrow to the far right of their name.
When prompted, confirm their upgrade to the role of "Captain."
You can downgrade by the same process.
Note: On every PRO Account there can be multiple "Captains," but there is only one originating "Captain-Admin." Captains have all the rights of the Captain-Admin, but with one difference: the Captain-Admin is the one with access the account's billing and credit card information and the one responsible for upgrading a group's member limit. If the Captain-Admin would like to share that role with another group member, they can do so by upgrading that member's status on their PRO Admin>Members page.