Re: Feeling like this is coming to an end
Don't get married, somethign is not right and marriage will only make things worse. You said she normally doesn't have friends. Was that because her world revolved around you but she finally met someone she enjoys spending time with and you don't understand why she is spending time with them and not you? Why are you guys not going out together as couples? If the friend's husband is going out it would make sense that you would go too? These sort of thing.....going out separately and not coming home or telling your partner when you will be home will bust a marriage. You should be going out as couples or she should go out with the girl. It's okay to drink with a friend and it is okay for her to have fun with a girlfriend but living with someone means being responsible to them as well....letting them know where you will be, about what time you think you might be home and telling them you will call if you will be later and then doing so. When there are unknowns trust becomes an issue.
You need to talk with your girlfriend and find out what exactly is going on. You cannot tell her what to do but the two of you can agree to ground rules. If she cannot play fair you need to walk now.
I have been married 32 years, early in our marriage my husband was this way. He wanted to hang with male and female "coworkers" that were not married or engaged and not include me. Would always have an excuse and didn't want to discuss this with me. He was even with an unmarried couple on our oldest childs 2nd birthday and he missed it because he was to busy running around with them. Fast forward a few years and again he found an unmarried couple that he was friends with and for the life of me I could not tolerate them then I discovered why. This male coworker was gracious enough to allow his "girlfriend" that turned wife to sleep with the landlord to pay the rent, then to sleep with the xhusband of hers to get them groceries. She was there to be shared and he kept this from me until I discovered it by another person's wife who's husband worked with my husband. When I she made comments about it in front of him and her husband he couldn't deny what was taking place.
You have to make the decision to as to if you are going to continue on and how far your relationship, but, know that things will change even more as time goes on the selfish behavior will continue to come out. I would also assume from my past experience there is something that she is hiding about these "friends" that she doesn't want you to be around to discover.