The lack of an internal messaging system within steemit is a problem.

The only way round this is to send the person a nominal amount of money (eg 0.001 SBD) through your wallet and add a message in the memo field.

They should receive a notification when the money reaches them, and hopefully they check it and read the message.

To send the money go to your Wallet, choose Steem Dollars, click on the drop down arrow to the right of the amount and choose Transfer. There you fill in the recipient account, the amount and the memo message.

Otherwise you can just comment on one of their current posts.