Membership not enabling free accounts when created.

As in the title, I have a membership option with a price of 0.00. It is intended people to sign up for the paid site content or just be an affiliate. Currently, when someone signs up under the affiliate-only option, the account is created, but is never set to 'active' automatically. It must be done by an administrator manually.

The user is informed that they have created an account, the admin receives an email stating so, and the default welcome message is sent to the user, but the account never activates.

How do I direct the system to automatically set the affiliate (free)accounts to 'active'?