The hard part of any startup is execution. How will this person contribute to your execution? What is your plan to finish beta 1 and get users to sign up? Will this person get you there sooner?
As with any project, you have to have a plan and milestones. Then you can start this person out with a small percentage. Give additional percentage as incentives to complete milestones.
Make sure you leave some behind to attract other people to join you!
