I don't think there is any fees on joining yobit, I didn't asked for any payment when i sign up either, and I never know there's any premium account there, though I may be mistaken. But there are some fees for exchanging from one coin to another and there are some fees on depositing and withdrawing with certain method. You can refer to the "fees" section that can be found under your account name on the top right corner.
quoting from yobits "add coin" page
Payment method: YobiCode
Listing type:
• Free - 0.00 btc - no guarantee
• Premium - 0.1 btc - 2 business day
• Exclusive - 0.5 btc - 1 business day + mail alert to all users
we decided to try to add another exchange for SOILcoin, and paid the .1BTC to yobit on february 9th. when 5 business days went by, i set up a support ticket. when it wasnt answered in a week, i added to that support ticket. ive posted against that support ticket 6 times now, in teh six weeks since we paid the premium listing fee, and have not once received a response. even when ive asked to just have the fee refunded and walk away from the idea of listing SOIL on their exchange, theres been no response in over a week. ive heard good things and bad things about yobit. i like their dice games and ridiculously numerous altcoin faucets. im not so impressed by the lack of response by support staff.