Depends totally on your relationship wit the girl.. If she sees you as 'hers' or if it has appeared that way to the other girls she will lose face in front of them and be angry / sad etc..
Now if you have a joking / flirty relationship and are already playing around with the other girls it may be obvious its a sex only type thing in which case they 'might' be ok with it.. Its a risk..
On my gogo trawls most of the girls know I have a regular GF so they know they are playing and having fun / parties / sex only.. Also I tend to take more than one if I do the pool party gogo thing which also stops the Gf aspect a bit.. its a ncie arrangement as now I pop in and the girls are friends with benefits.. I start to eye another girl up and they ask ' you like her.. You want me bring her over' etc.. They know with my tatses I will probably take the bunch anyway so they are getting a BF..
You might suggest the 3 some with her and a mate if you really want the mate and she is coming on too strong as a GF.. But dont do the GF then GF+ friend then swap to the friend directly.. Looks like GF1 cant perform as well as GF2 and thats another way to insult..
Basically it would be much easier to just go somewhere else
