I'm not sure we need to put more effort into making new users feel welcome. It's a priority for churches because their entire business model depends on generating warm, fuzzy feelings in their members. Responding "Welcome to LessWrong!" to a new user's first comment is fine, IMO.

group project goodness = U(project) / E(social friction),

Why is social friction an expectation, but not utility? Why division instead of subtraction? This equation should have been a sentence. I also don't see why you've singled out "social friction" as the biggest drawback for community-building projects.