co-worker; partner; collaborator
Refers to someone who works together with others on a shared task or project. Neutral term without strong hierarchical nuance.
He joined this project as my co-worker.
I solved the problem in cooperation with my partner.
協力者 also means 'collaborator' but often implies a helper or supporter, whereas 共同者 suggests equal partnership in a joint endeavor.
Compound of 共同 (kyōdō, 'joint, cooperative') + 者 (sha, 'person').