is it possible to know uninque ip addresses of node having same group id(multicasting group)
What you are looking for in multicast terms are the members of a given group.
The 'official' way is to multicast a membership query to the group, to which each local member (on the same network) will respond. Takes a bit of network programming to create the query and parse the responses.
The 'unofficial' way is to simply ICMP ping the group address. Most members will respond. Something like ping -I eth0 -t 1 -c 10 188.8.131.52