To allow your members to sign in, using their Yahoo! credentials, to access your Ning Network, head to the Sign up/Sign in page under the Members section of your My Network tab. Check the boxes next to the Yahoo service in order to enable sign up and sign in with this service.
Click “Save”. Now members who wish to sign in with their login information from Yahoo! will be able to do so. The first time they sign up, your members will see a window pop up, informing them that your network is requiring information from their account. To accept this connection, they can click the “Allow” button.
If you wish to disable either service at any point, go back to your Sign up/Sign in page under the Members section in your dashboard. Uncheck the boxes under Sign up/Sign in. A lightbox will pop up, alerting you to how many members currently use this service, and that removing this function will prevent them from being able to sign in with these credentials.
Re-enabling this function at a later point will allow these members to reconnect with their Yahoo! credentials.