A Fabric network member is nothing but a user on the Blockchain network who indicates an organization.
The example given below indicates that in order for a transaction to be endorsed and sent to the orderer, a user from each organization needs to endorse.
AND('Org1.member', 'Org2.member', 'Org3.member') requests 1 signature from each of the three principals
Admins can add and remove members from the network and modify member settings.
Peers may be endorsing or regular (a peer that does not endorse but commits transactions).
Clients are usually organizations that invoke smart contracts on the Blockchain network.