You don't configure multicast address anywhere.
Some box sends traffic out with multicast destination, this will automatically get L2 multicast address on the link.
This L2 multicast address is normally broadcasted in L2, unless there are some specific features enabled, like IGMP snooping to stop the flooding.
If IGMP snooping is enabled, then the L2 switch will learn which ports want to receive the multicast (they do IGMP join to the multicast group) and will not flood traffic to other than interested ports.
If L3 is involved, then you'll learn via PIM which ports are interested in receiving the multicast.
I can recommend this book for multicast basics, it's not JNPR specific.
If you don't know which addresses to use, if you have 2byte ASN, you should use
18.104.22.168/8 GLOP block (with your ASN in it) if you don't have 2byte ASN your best bet is 22.214.171.124/8 which can be thought has RFC1918 (e.g. 10.0.0.0/8) for multicast.