My partner and I have been together 7 years, are ENM and have had a few group parties, and our relationship is stronger and more loving than anyone we know.

Our NUMBER ONE rule for threesomes and outside partners is that they cannot be anyone we know from before we open the relationship (5 years now so generally not an issue any more).

Generally speaking, it just complicates things when you seek out old flames or add sex to established friendships.

With that said, you literally just text them and say "hey if you were serious about wanting to have some fun together, me and ___ are definitely interested. If not, no worries!"

The two most underrated aspects of open relationships are 1) nobody has time for games or riddles and 2) it is actually pretty fucking cool to discuss your sex life and get advice from your best friend and most important sexual partner.