You should have them send you the completed project folder. You might have to tweak the provisioning profile settings to get it to run for you, but you will be able to build it and thoroughly test it yourself, then archive it (Product menu → Archive) and upload it yourself. This will also make sure that you have a copy of the code—very important for ensuring that you remain in control of your app.
And they don't need to be admins on your development team. As a matter of fact, I wouldn't recommend giving an outsourced contractor any role higher than Developer. You should give them access to your development signing identities and provisioning profiles so they can develop for stuff like iCloud and Game Center, but they don't need to be able to actually submit the app.
Just so you know, though, I'm not an expert on the whole outsourced-development thing; right now I work alone and have no experience dealing with this, but I hope this will steer you in the right direction.