I agree with Pam but wanted to add that most often VAs will subcontract to/work with virtual assistants they already know through the forums and social networking sites. The more active you are on a site like VAF the more you'll become familiar with other VAs who might complement your own service offerings and/or could serve as backup if you were to get busy.
There are also plenty of multi-VA teams who's members network here on VAF. They generally discuss business operations and the ins and outs of working with other VAs in the
Multi-Virtual Assistant Businesses Forum. You'll definitely find some great information related to growing a team in there.