Ben Vollmer has written an excellent article describing how to create a pseudo many-to-many relationship within MSCRM 3.0.

In case you didn't know, MSCRM 3.0 doesn't natively support many-to-many, or one-to-one relationships and as Ben states: it's "One of the things that has driven a few newer partners who are used to other CRM Systems bonkers…"

Check out Ben's article should you need to create a many-to-many relationship.