Should I tell a guy I like (with a girlfriend) how I feel?

Usually I would say no, but my friend is in the same situation and she likes the guy so much that I can't help but to encourage her sort of. I mean its not like the guy is married. I suppose its okay to let him know how you feel, but dont try to seduce him, tell him how you feel and let him make his own decision. And if he doesn't choose you, don't push the issue, just let him go.