When a newcomer arrives at your community, what have they heard about the community so far?
This affects the messages they need to receive.
Have they heard about something specific happening within the community?
Have they heard about the community as a place where talented people in their sector go?
Have they heard about the community as a place that's fun to hang out in?
If they have heard the community is great and they should join, then converting into members is about reinforcing this perspective. You highlight current successes and lead members to the registration form.
If they have vaguely heard about the community, then you need to educate them and make a member more excited about the community before they hit the registration form.
If they have heard about a specific resource or event taking place within the community, then listing upcoming events on the non-registered landing page of that community matters.
You might want to create special non-member pages for those visiting from specific channels (e.g. search, twitter etc..)
It's important to uncover the mindset of members when they visit the community for the first time.
Broadly we see two types of newcomers Those already primed to join and those visiting out of curiosity. Converting the former requires reinforcement of existing messages. Converting the latter requires a little more persuasive messages.
If you can survey newcomers randomly, that works fine. If not, look at the origins of traffic (or ask how they heard about the community) and make an educated guess.