Switch into Admin mode for your organisation.
Go to your dashboard and select to Manage Members
Here you can view all standard members and Admins
Select 'Invite members' from the navigation
Then select 'Add Manually'
Enter email addresses for the admins you wish to invite. You will need to use the Trash icon to remove any extra rows you are not using.
To finish click "Invite users".
Alternatively you can click the copy URL button and share than link via your own email system.
This will invite the person as a standard member. Once they have signed up you can co back to your members list, and using the 3 dots menu on the right of he member you want to promote to an Admin - select Make Admin from the dropdown sub-menu.