I hold an IPC license; do I also need to hold a U.S. Ski & Snowboard membership?

If you hold an Alpine IPC license, then yes, you also need to hold an Alpine U.S. Ski & Snowboard membership.

If you hold an Snowboard IPC license, then no, you do not need to hold an Snowboard U.S. Ski & Snowboard membership.  However, you do need to hold an Snowboard USASA membership.