1) You have to create a bank account so they can transfer Tu. You can do the same by filling out the transfer option there once you make a bank account. To do so, go to Reiflem bank by clicking the amount of Tu you have in the 'today' box at the left hand corner.
2) To deposit, withdraw, transfer, and receive Tu from other users. There is no interest.
3) I believe you can't anymore, as battling is down and leveling has to do with that. I'm not too sure though, as when battling was up, I wasn't a user. XD
4) You can, either by putting it in your rancher shop or clicking 'trade pets' at the navigation bar under play. The other person needs to get what is known as a junk or trade pet- usually a cheaply colored pet that is virtually worthless because of its name. Then you type in their username and set a trade with the junk or trade pet using the pet you want to give them.
Welcome to Res, by the way. ;)