While many entrepreneurs start out as the only employees of their company, the smart ones hire a handful of freelancers to help them do more faster.
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I've worked for a number of Marketing/Strategy/PR and even Design firms that work as virtual agencies. They usually have a few people on staff and everyone else is freelance. They don't feed me work for a cut, they usually hire me to work on a project that they are pitching, or have.
Here's some things you might consider
Account/Project Manager, someone needs to be at the center of the project, to do budgets, calendars, coordinate meetings etc
Meeting space (preferably with doughnuts)
Consistent conference call number, fussing with conference calls on a regular phone is annoying as heck
Online calendar (google calendar is great)
Collaborative file storage so that everyone has access to work files (dropbox seems to be the standard)
Pantone books (it's surprising how often you need them)
Standard set of paperwork that everyone fills out before they work for you