Most people probably know this however i'm sure there are those that don't.
You gain full EXP from any quests and monster kills without being present and just being in "town"
Obviously there are better ways to level secondary accounts and friends faster.
However if you just want to do a run through of an act or mission, you can have a friend or second account just sit afk in town and they will recieve experience for any monster kills and quests that you complete.