Do girls act like their girlfriends?

I wouldn't say that is always the case, but if your girlfriend is very impressionable and influenced by her friends a lot, then maybe. Now, hopefully, she wouldn't act in a way where she would be hanging on other guys. I can see maybe acting silly and dancing crazy in a club but I can't see how hanging on different guys would be a trait they would share, especially if she cares for you.

No you could not.

While they do act the same in some things because you are bound to end up acting like each other. There are things I do that my friends have told me they would never do; and I know they would never do. But same goes with me and them. They do stuff I would never do in a million years but that doesn't mean we can't be friends.

Ask yourself.

Do you do everything your friends do?
Would you want a girl to judge you on what your friends do?

Then why should you judge her on them?

I hope this makes sense :]

This is a tricky question to answer.
Mostly I would say no because I myself have friends like this and I don't do this. Chiefly though I think this depends on your relationship - is it strong or are you guys on the rocks constantly.
When a girl feels secure and happy in a relationship she will not hang all over other guys. She will not be interested to at all. On the other hand if you have a sketchy relationship your girlfriend might just follow her friends because they are enjoying themselves and she's not been lately so she might just hang out with a new face. In other words. If you aren't making her feel cared for, giving her attention, and over all just showing her that you are loyal to her only. Then she will be looking for that when she is out having fun. In other words it is more important that you give her what she needs from a relationship or she might follow her friends and look for someone to fill that void when they are out trying to have fun.