This has never happened to me, but one of my friends is in this situation and i though it was a good topic. These situations can be very difficult and need to be handled maturely.
So your crush has a girlfriend? I really hope your crush is not a player because that would make everything super difficult. If he has a girlfriend and you are talking to him be careful.
Some girls are very protective of their boyfriends and they don't want other girls involved.
My best friend has already been messaged about it. The guy is not allowed to talk to her on anything or be seen with my friend. Don't get yourself in this situation because it sucks.
Be friendly to his girlfriend and don't try too hard because the girlfriend will notice. If they try to message you rude things just ignore them and avoid them when you see them.
Hopefully this isn't common but i know it happens. It can suck when your crush has a girlfriend.
Sorry this was so short but i'm out of ideas and i don't know what to write. Maybe i'll have a part 2.
Be sure to leave me some ideas and to ask me if you need advice :))