Either way works, but ultimately, the teacher is responsible for adding the student before the lesson begins.

The best teachers add me on Skype the day before, ask me what I want to do for the lesson tomorrow, and send me materials to preview (videos, exercises, etc). They get a big juicy 10-lesson package from me.

The worst teachers don't add me before the lesson, I add them an hour before, they don't accept, and I waste 5-10 minutes of my 1-hour lesson trying to get them to accept my Skype invitation. I never see them again and they get a big ugly rating that is definitely not a 5.

Usually I add them first unless they choose to be the first one to add me