See those chairs? That's how you visit other players taverns. See "FInd Friends"? If you click on that you'll be shown a bunch of random players. You can invite until you have 80 friends or invites. You can accept upto 140 friends
In your tavern you'll need to
click on the tavern and clear the seats whenever it's full from your friends visiting. Once you've got enough silver you'll be able to buy a boost or upgrade the tavern