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.
do you know some sources about creation of accounts using blockchain api? although I doubt we can afford to pay all the fees for creating of a certain amount of users. especially not when assuming the user base grows at a fast pace :-/
steemit verification process takes currently takes too much time
Can you message me on steem.chat or Discord?
edit: ok I managed to find you :-)