You can't "make" her do anything, and why would you want to anyway? Does it turn you on to think of her with another guy, or do you hope that if she gets another boyfriend first then she'll be ok with you having another girlfriend? What is this about?
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.