Active records - "smart" select

[eluser]ibnclaudius[/eluser]
I have three tables: users(id,name), users_communities(user_id,community_id), communities(id,name)

I know how to select the communities the user doesn't have, but I want a more efficient result...

Here's my idea, I'll show an example.

User A has these communities:
PHP,JAVA,ORACLE

User B:
PHP,MYSQL,JAVASCRIPT

User C:
PHP,CODEIGNITER,SOCCER

User D:
PHP,CODEIGNITER,MYSQL

Let's say I want to show to user A the top 2 communities he may want. In this case would be CODEIGNITER and MYSQL.

What I did?
Select all common users in communities that I have.
Select the 2 most common communities in these users.

I'm pretty sure that should be something much more complex, but this is the beginning...