Being at the same place at the same time (not planned) is obviously just pure coincidence. Therefore, I would have no problem going up to the group to say hello. Just be genuine. State the truth, that you saw them standing there, and didn't want to be rude, so you came over to say hello. That may lead to further conversation. However, if it doesn't, simply continue doing what you planned on doing before you even knew they were there.
Let your life be driven with purpose!