I would use it in either situation. I'm excited to see you tomorrow, it has been too long! (before I see you) I'm excited to see you here having fun! (when I see you)

You would usually say it before you see the person, but sometimes if I haven't seen someone in a long time, I will say "I'm excited to see you" as we greet each other. Or I say, "I'm so happy to see you (again)!" But either would make sense and I don't think would be weird if you're close with the person.