1. Make sure you are using Google+ as the page that you want to add admins to. Click on the little gear icon in the top right corner. Then click Google+ Settings.
2. Once you get to the settings page, click on Managers on the left hand side
The person who originally made the page is the sole owner. If you want to transfer complete ownership to someone you have to make sure they are a manager of the page first and then you can transfer it over. If you click the "learn more" link there is a bunch of extra info that might be useful to you.
Just that simple!