Can I sign up if I am already a part of an ad network?

Yes. However, we do have some guidelines as to how non-Google ads may be displayed along with Google ads. We do allow affiliate or limited-text links.

If your ad network already displays Google ads on your pages, you can still join AdSense and run ads on your site through our program. We allow publishers to display up to three standard Google ad units on any webpage.

Keep in mind that it's your responsibility to work out any contractual issues that may arise with your ad network as a result of serving Google ads directly.