I saw Rent there, Benny tour final stop, but that was ten years ago, and my memory of the crowds is getting confused with the crowds at the Shubert in Chicago, which was the final midwest stop. I want to say the stage door is internal at the Orpheum, but it may be whatever door is to the right if you're facing the main entrance. Either way, hanging out on the front sidewalk on Market is what you want to do. There will almost certainly be a note taped to the stage door saying "Stage Door", with the LM logo - the cast and crew don't know where the stage door is, either, so it gets labeled for their benefit. If the stage door is internal, there may not be a label in a place you will see it when exploring outside.
One caveat: I don't know if downtown SF has gotten any better in the past 10 years. The homeless population in this area used to be fairly aggressive, and being down there alone after dark was not advised. With Rent, this wasn't a big problem, as the crowds were huge every night, but LM won't attract the same numbers. This will also keep the actors moving quickly, probably, as they don't want to get hassled, either. Be aware of your surroundings, not just aware of actors. If you go after a matinee, thus between shows, not everyone may come out; at night, everyone will have to leave eventually, but be careful. I've not had problems anywhere, but the Orpheum is the only place I was ever warned about the potential, and that warning came from local people who I've stage doored with at the Curran with no problems and no warnings. But, summer 2001 is quite a while ago.
Varela is cool, but I do not recommend glomping anyone you've never met before. If a stranger glomped you, you might find it weird, right? Varela's cool and likely to say yes if you ask if you can give him a hug. Just be polite with everyone, and you'll have a good experience.
What kind of literature and what kind of life is the same question. - Tom Stoppard