I've been living with both of my life partners together for over a year now-- along with a bunch of other people. We cohouse in a townhouse together (sharing rent, utilities, groceries, chores, etc.) My two partners and I see each other daily-- they each have me overnight a few nights a week, and how we spend our days (after work/school/other commitments) is up for grabs.
I'm lucky enough that my girlfriend, my friend with benefits, and my lover-to-be (cheers to a complicated life!) are actually all moving in with us, starting this summer.
Up to this point, my friend with benefits has visited us on weekends and gotten an overnight or two each time. My girlfriend's been in college (graduating next month!) and visiting between semesters for a month or two. My lover-to-be and I have just been getting phone dates-- he's been living a few hours away, and we haven't spent much time together literally in years.
Now, we're actually in the process of revising our schedules-- particularly our sleeping ones, so that our world will still function.
As for a question about primaries getting a say in all of this... Everybody (not just my lovers) got a say on people moving in at all. Scheduling is a collective responsibility.
Living the dream...