Oooh well, there's about 97 ways of doing this.
If its an AI group with a getin waypoint, you could write
grp1loaded=true
in the On activation field of the waypoint. When the group are all in the variable grp1loaded will be set to true and then your trigger, which has this in the condition field,
Condition: grp1loaded
will fire. If it's a player led group, then you might have a trigger like this
Condition: count units grp1 == "_x in lorry1" count units grp1
Syntax not guaranteed.