Yes, they even have special dialogues when you talk to them about it
This is how it works:
Each girl has an affection stat with each other (in the files they show up like RogueX.LikeKitty/Gwen/Laura/etc)
To gain affection the girls need to do activities together, that means studying, showering, threeway dating, Threesomes, training, etc.
Then (this is very important) the girls need to be pent up, meaning that you have to decline their advances when they get horny, only then they will actively start "hanging out" with other girls. You can then walk in on them or give them a call while they are in the middle of the act.
Frankly I have mixed opinions on this, it lends itself to NTRshit and If Oni ever decides to add dudes then this system is bound to lead to those scenarios since he wouldn't need to do much to add them.
Lesbian relationships are fake so it doesn't matter when it's girls doing it tho.