DIfference Multicast and Unicast Routing

In unicast, there is always a route in the unicast routing table for any (valid) destination
In multicast, a multicast route to a receiver is created only when that receivers joins a multicast group.
I.e., the multicast routing tables in routes are being manipulated by multicast receivers !!!

How to become part of the Multicast network ?

Host computers are typically connected to a (Ethernet, FDDI) LAN
In order to provide multicast on a (broadcast) LAN, a multicast enabled router must be present on that LAN.
Such a multicast enabled router can process IGMP (Internet Group Management Protocol) messages
A host on a broadcast LAN (Ethernet, Token ring) that wants to join a multicast group will announce their multicast group memberships to the multicast router using a special IGMP-JOIN protocol message:

The multicast enable router receives the IGMP join message and forward special route update messages to other nodes to extend the multicast forwarding tree to accommodate new receivers on its LAN.