Do you only get ONE official Girlfriend ???(and the rest of the girls are friend with benefits)?
Or do they all become your official girlfriends?
Eh, if you slow down early on, you can straight up asking for a girl, who want to become your girlfriend to be poliamorious. It's better option than alternative, if you want to bed others too, but require farming 600 trust. Depending on how you answered in quests (and if you even triggered optional ones) it's can be "just a couple of in game days" or whole weeks of extra grind.
You also need to ensure that your current GF is friendly with harem candidate, and confirm intention, otherwise she will see it as betrayal.
This requires them to do stuff together many many times. Like studing together, training together, money nights, BBQ party (which require them already having high enough love stat) or pool (which have two issues, main of which is that girls will get tan pretty fast this way so if you don't like tan lines then this option isn't good).
The scores the girls get for this activities are basically same but "treshhold" their require to change relationship status is different. For example, it's much faster to make Rogue and Laura being on friendly terms than Jean and Laura or Rogue and Jean. More so, one girl can consider another a friend while vice-verse not being true.