To add admins and ambassadors to your Community, you should first make sure that they are active as admin users or buddies in your admin dashboard.
If they are already added as an admin user, or a buddy, then you can add them individually to each Community you would like them to be a part of.
--> Go to your Community tab in your admin dashboard, and click on the Community in question
--> You will find yourself in the Users tab for that Community
--> Type or paste the email addresses of the admins or ambassadors/buddies that you'd like to add to this Community
--> Click Review Invites
--> Click Send invite with email
Your admins and ambassadors will receive an automated email asking them to set a password, and they will then be able to log into your Community. As they are existing users, they simply need to log in as existing users, rather than creating a new account.