>>3731practical example:
friend: come to my dorm and watch a movie
response: ask to bring a friend, or your gf maybe, or decline
another example:
friend: let's go grab lunch
response: no problem assuming this is public
it depends also on your current girlfriend and how suspicious she is. Best to tell her what you're doing and see if she has a positive response. If your current partner is too jealous then you will probably either have to choose or develop your current relationship further to get this level of trust if possible.