Use the unitnames, not the group name to join grpnull. I never heard of a group joining a grpnull all at once. If the helo is named helo1, then use-
[helo1d,helo1g] join grpnull. The 'd' & 'g' are for driver and gunner respectively. Get the idear ? Rejoin them to each other the same way.
[helo1g] join helo1d; helo1d move getmarkerpos blah blah....