I had to rebuild the library to get IGMP work. Contrary to what the documentation implies it is not enabled by default. If not part of the library, you will get an error that it can't find "SOL_IGMP_CALL" when trying to use "SOL_IGMP" to set socket options (assuming you are using BSD model). Open project for your development board (rtsc_m52259evb) find user_config.h and add "#define RTCSCFG_ENABLE_IGMP 1" in the "board-specific RTCS settings" section.
I can then send and receive multicast messages. Having said that, I don't seem to be seeing any membership reports being sent out. More to come as I'm still evaluating this...
PS, this logically belongs in the RTCS section of this Forum
Bill