how to get 'join my facebook group page' button on my homepage?

how do i get a facebook button on my website’s homepage – and it perhaps says “join our facebook group page” or something similar, and then clicking on it will take readers to my already existing facebook group page? (i suppose for people who are not yet facebook members, i want it to take them to a place where they can a) see my group page and b) join facebook, so they CAN join my group page. ???

i looked thru plug ins, and don’t see something that. i’m baffled about this. thanks in advance …