Just tell them. If they treat you different or put you down, then they were never your friends to begin with. Find people who accept you for you.

I used to know a fellow who was Bi, and I even had a few gay bosses in my time. I held nothing against them as I could care less what their sexual preference is. Some people may look at you different, but don't let it bother you.

You have to think though, do your friends really need to know that you're bi? If your friends aren't into playing for both teams, you'd do it out of respect not to talk about how good looking you think a certain guy is. That's how I see it anyways.

Just be yourself.