[unitname,unitname,unitname,...] join yourgroup
[unitname,unitname,unitname,...] being an array of units you want to join to the group and
yourgroup being the groupname in which you want them to join
to get a groupname you should for example put
yourgroup = group this
into the leader unit's init field.
That's the way you join units to groups.
If you want them to just follow a group without joining it you can attach a move waypoint by selecting waypoints, selecting the units and then double clicking right on top of the leader unit of the group which you want to be followed.
Or if you'r familiar with scripting you might want to make a script with something like this
#loop
(leader mygroup) move getpos (leader theothergroup)
~20
goto "loop"