Co- (together, with)
Co-
Together; with someone else.
Rohan and Fatima co-wrote the class newsletter.
Cooperate
To work together towards the same goal.
The two teams cooperated to clean the beach.
Co-author
A person who writes a book with someone.
Ananya is the co-author of a short story collection.
Remember: Co- = together. Some words take a hyphen (co-author), others often don't (cooperate); dictionaries differ. Say koh-OP-uh-rate: two separate o sounds, not 'coo'.