I would wave to them while they were looking at me, from a distance. If they returned my wave and motioned to me, then I would go and join them, even if only for a couple of minutes. After all, they have plans together (and I wasn't part of those plans), and *I* have plans (or I wouldn't be in that spot in the first place), and I didn't invite *them* to be with *me.*.
If they don't pay attention to you even though you think they saw you, don't overthink it. Just go about your business. The next time you see the one with whom you think you maid eye contact, ask him/her by saying, "I saw you all the other day at ____. I would have come over, but I was on my way to _____ and I know you all had plans, too. Hope it was a good time for you all!"
The biggest mistake you can make here is to overthink this. Let it go.