Both.
The primary decision maker in this process is the doctor. The Managing Networks have two objectives in this process. Offering a large network to their insured and keeping their costs low. They try to find doctors who will accept the payments amounts they determine appropriate and have little or no incentive to decline any doctor who will accept their pricing.
The doctor may not feel that the amount of compensation the Managing Network is offering is sufficient so they will not join the network. Other factors that the doctor considers include how long it takes to get paid by the insurance company, how much business they currently have and how will a network participation decision affect their practice.
Consequently in a vast majority of the cases, the determination to participate in any specific network rests with the doctor.