I don't want to go on my friends birthday trip
Why would you not want to go?
I feel like a loser, I can't handle a group of people.
It's understandable to not want to go on a trip. You are within your right to not go on a trip that might make you uncomfortable. Let your friend know though before making that decision.
I hate the idea that I'm a loser.
We, sometimes feel that way once in a while. It's normal. You should go hiking or something and breath the fresh air to get a new sense of yourself.