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.