Board coins are standard BEP-20 tokens, so you can freely transfer them like any other tokens. The address of each board's coin contract can be found in the board's menu at the three dots on that board's page.
You can also send your board coins to any MAIN user if this user has connected his BSC address to his MAIN account using the internal interface of the site. To do this, go to the board page, click on the Buy coins button and go to the Send tab. Select the username of the user you want to send coins to, enter the amount of coins you want to send and click on the Send button. Sign the transaction in your wallet.
Coins will be sent to the address associated with this MAIN account.