I did this successfully with my agency http://bootstrapheroes.com/ https://experts.shopify.com/socialproof-it we grew from a team of two to 10+. The biggest lesson I learned is if you want to create an agency you need to become one of the top experts in a specific area that is relatively new and growing like crazy. In new areas their is no experts so very easy in the beginning to become one or fake it until you are one. For me it was becoming one of the top Shopify app development experts. If I had tried to start an agency in a mature market like development for Magento/ Drupal sites I would have had a way harder time for their is lots of agencies with great track records in these areas.