It changes occasionally, but you can do it one of two ways:

• With delegation (cost would be like 0.2 STEEM + 65k VESTS delegated)
• Without delegation (last I knew this method was like 20 STEEM)

They also shouldn't have to share the private keys. The create account API call only needs public keys. So having them generate the keys safely offline and sharing the public keys creates a very safe (and potentially offline) method of registration.

I need to dig into account creation services at some point here soon for chainBB, I'm hoping to pair it with Vessel :)