In fabric, there is an admin, he manages the entities (the users, the peers..) that will be the part of the system. The admin also enrol an admin user for each organization, or he enrols the peers and users for each organization.
After enrolling each entity, the registry is done. When you do a registry aginst the certificate authority(CA), the CA generates a pair of keys, which is issued by the CA root. The user and the peer are thus enrolled to the CA. You need a peer to communicate with other peers and the orderer.
Also, you need a user to call the peer to genrate the transactions. When you create a channel, you have to define who is going to take part on it, i.e. you define which peers are going to interact among them. if your peer is not defined in the channel configuration, the acces for it will be restricted.