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From the Model Mayhem FAQ:

How does registration and the approval process work?
Once you’ve registered your member account, it must meet all of our requirements in order to be approved to access the site. These requirements are clearly laid out here: http://www.modelmayhem.com/help/rules/membership-requirements

You will need to submit a biography about yourself – at least 100 characters – and at least 4 pictures that meet the requirements for your chosen membership type. You can enter these things from your My Stuff.

After these things have been submitted, we send your submission to a selected team of members from all fields in the modeling industry to review your profile and ensure it meets our membership requirements.