We've trained several clients to have superuser groups in different regions. I would suggest you have regular catch-ups to share lessons learnt, what worked successfully, what failed, best practices, ideas, Q&As, etc. I would also recommend you document everything and make sure everyone has access to it. These documents will need to be reviewed and improved regularly.
Also, maybe now that Marketo offers different specialisations each of you could focus on specialising on a specific area and share the knowledge to other team team members.
Hope this helps you!
Depending on your company's size, you could always create a marketing knowledgebase and allow people to contribute to it. They could include their logic for smart lists or links directly into Marketo for people to see and replicate. They would function as both role models within your company, additonal Marketo experts *and* you could share regional campaign wins for others to copy.
I have a few power users at this company and a past company, the only ways I've been able to identify and encourage them is simply from working closely with them in a support role and finding who has both interest and innate talent with the logic that comes with marketing automation. Then you can always create a special group to pull them together, offer additional training/mentoring.