Refers to a person who jointly establishes a company or organization with others.
He is a co-founder of that startup.
It is important to share a vision with your co-founders.
創業者 means 'founder' (sole founder), while 共同創業者 specifically means 'co-founder' (one of multiple founders).
Compound of 共同 (kyōdō, 'joint, cooperative') + 創業者 (sōgyōsha, 'founder').