IGMP is not yet supported, and there are no plans to do so yet.
But it shouldn't be a problem to add some hooks in the code so that you can write you own IGMP handler.
If you want, write me at "h.tibosch" on freertos.org and we'll see what needs to be done. It can't be too much work :-)
Thanks for getting back with me. Some industrial protocols need this support. I am going to test Ethernet/IP with the stack and for full implementation IGMP is needed. But I can test as is for now.