General Use:
* cooperate: to work together towards a common goal
* coexist: to live or exist together peacefully
* coworker: someone who works with you
* cofounder: one of the people who starts a company or organization
* coauthor: one of the people who writes a book or article
* cohost: one of the people who presents a show or event
* cosigner: someone who signs a document along with another person
* cohabit: to live together as a couple
* coeducational: having both boys and girls
* coordinate: to organize different parts of a project
* cohesive: sticking together, united
* coherent: logical and understandable
* coincidence: something happening by chance
More Specific:
* coeducational: having both boys and girls
* co-op: a cooperative, a business owned by its workers
* co-pay: the amount you pay for a medical service
* co-sign: to agree to be responsible for a debt
* co-pilot: a pilot who assists the main pilot
* co-production: a project made in collaboration with others
* co-star: an actor who stars in a film or play with another actor
* co-worker: a person who works with another person
* co-sponsor: to share the responsibility of sponsoring something
* co-author: to write a book or article with someone else
Less Common:
* coalesce: to come together and form a single unit
* cogency: the quality of being clear, logical, and convincing
* cogent: clear, logical, and convincing
* cognate: related in origin
* cognizant: aware of something
* cohesion: the state of being united or sticking together
* cohort: a group of people who share a common characteristic
* comity: courtesy and mutual respect
* commemorate: to remember and celebrate an event
* commendable: worthy of praise
* commensurate: corresponding in size or degree
This is not an exhaustive list, but it should give you a good starting point for finding words that begin with the prefix "co-."