Hi, I'm not sure what the limit is (I suspect 256 characters). We had a similar issue with our everybody net group (all the machines on the net work). I think the way around this is a hurd of sub groups which are within the limit and the master group referencing the subgroups rather than the members directly.
we've since removed this group, so I can't verify this, and our NIS is run on Solaris machines so YMMV... -Jon