Right now the best option is probably to send them to signup.steemit.com. They will have to go through the verification process (which takes some time) but it is free for them to sign up because Steemit pays the account creation fee and gives them an initial delegation of Steem Power.
If you have the ability to pay the account creation fee (currently 0.1 STEEM per account) and delegate SP to each account (currently around 15 SP per account) then there are some other options available using the blockchain API.
RE: Dev Portal Update: New Steem Developer Resources