One feature of the admin you are bound to notice is that on the Organizations page, some of the listed orgs have a little green dot next to them and others have a grey one. This is active verse inactive organizations!
The active verse inactive organization distinction is made within the settings icon of each org. You can find that directly to the right of their information on the settings page.
In order to make an organization active, you just have to set the contract start & end dates! It's an easy is that!
Why is this important? Only active organizations will show up in the browse organizations dropdown.