You're not doing it completely wrong, just wrong enough.
The first thing to get clear in your head is that, in scripting terms, a group is nothing like a unit. (In the same way that a banana is nothing like photocopier.) Some scripting commands work on groups, and some on units. A few clever ones can work on both, but they are the exception not the rule.
In this case you have it basically right except that you are trying to make group do an action. This is like ordering a banana to make a copy of your CV. Actions are done by units.
You have named the group correctly, but you need to order the units to eject individually. There are several ways to do this, here's one cut and pasted from a script file one of my working missions.
_group=units alpha
_i=0
#lop
_loon= _group select _i
_loon action ["eject",chopper1]
unAssignVehicle _loon
_i=_i+1
? _i >=8 : exit
~0.5
goto "lop"
exit
This makes them jump at half second intervals so they don't get tangled up. Make sure they are high enough for the chutes to deploy. If you don't unassign them from the vehicle they try to get back in it.
There are various paradrop scripts in the Ed Depot, mostly for HALO and other fancy stuff.