@Haymaker
Would this work for you?
_group addWaypoint [_position, 0] setWaypointScript "\x\cba\addons\ai\fnc_waypointGarrison.sqf []";
alternative:
[_group, _position] execVM "\x\cba\addons\ai\fnc_waypointGarrison.sqf";
ref: https://github.com/CBATeam/CBA_A3/pull/851#issuecomment-353932357
The script is not meant to be a callable function. It is meant to be attached as waypoint script of the GARRISION scripted waypoint in the editor.
I'd gladly add this to the functions header.