The former, ofc
Though I suppose sharing could just mean that they're willing to allow you sexual relationships with other women WITHOUT participating in any actual sexual encounter with the other woman themselves, i.e. no threesome etc.
Funny, Brit doesn't come off to me as being into that kind of stuff. But, I usually find out I know nothing about these characters even though I play the game
I see her as being one of those aforementioned young-and-impressionable types that experiment with sharing, but find out they don't like it in the end. That's what I'm gambling on, wonder if I'll be right or dead-wrong as usual
Nicki might be a candidate for a lot of people, but I'm honestly not sure about that since she's a pretty complex character with seemingly contradictory thoughts and behaviors. I'd absolutely bet that you could convince her to let you have relationships with other women, even to participate in the odd threesome, because she's one of the, if not THE, most easily-influenced characters in the game. But, I have a feeling that she will be happy and ok on the outside, but hurting and resentful on the inside. And she'll probably let it fester too because she'll continually want to do w/e makes MC happy, yet will feel doubt and guilt towards herself at any desire to put a stop to it or to say no. She's clearly in love with MC, and if she finally gets what she's clearly wanted this whole time, a romance with him, she will probably be petrified at the idea of 'ruining' it after everything that finally lead up to it.
Again, this is just my crappy analysis of her character, I'm sure I don't actually understand her at all. It's also possible that by the time sharing comes up in the game, Nicki will have gained enough confidence and self-awareness to nip this sort of thing in the bud before it gets out of hand. So, I guess in that case, my analysis would apply to old Nicki, but not to potential reborn Nicki after her inevitable arc.