When I got close to a pair of people, about 70% of the time one of them would make eye contact with me, so I'd introduce myself. The other 30% of the time they stayed intensely focused on their conversation, so I'd move on.
Just remember that everyone else at these events is there to meet other people too. They WANT to mingle, and by introducing yourself, you're helping them do so.
I am so glad you found this story helpful. This advice made a world of difference to me, and frankly I'm still surprised it worked so well. I hope it helps others too!