You could notice different availability adjustments during the offer adding process. Also the vast majority of offers is added with the "By request" availability, which means, your affiliates have to be approved prior getting tracking links and promoting such an offer.
Affiliate will see such offers with a lock icon in the list:
The same in offer details with a "Send request" option:
Once the request is sent by an affiliate, it will appear at the bottom of the admin/managers dashboard at the Offer Requests tab:
Now you need just hover on the request and use appropriate buttons to approve or deny it.