Another way is to use a flavor like RPCSEC_GSS which doesn't send group identifiers. Instead, it lets the NFS server decide what groups the user is in (server determining access controls; what a novel concept!) based on the local /etc/group file or group tables in NIS or LDAP. Since there is no group id array in the RPC message, only internal NFS server limitations get in the way. NetApp's ONTAP server for example supports 32 supplemental group identifiers. Last I checked Solaris was either unlimited or up to 64, but it was subject to a tunable parameter. A side benefit of RPCSEC_GSS, if used over something like Kerberos V5 or public key certificates, gives you true authentication.