This is a very minor modification to the Carrier - Hangar ability that illustrates what you can do using it. Multiple unselectable/uncommandable units of varying type follow the hero and attack targets for you. All kills accumulated by the group are credited to the Hero.

The attached example map was broke by a patch, but gives a general idea on how it was accomplished

BASIC HOW TO: It's simple to add units to a party.

Duplicate the 'Carrier - Hangar' ability (Don't check any of the boxes)
Double click 'Ability - Info +'
Double click 'Ammo 01' (For first unit that follows, Ammo 02 for 2nd unit, etc)
Set 'Info - Count - Start' to how many of that unit you want to follow.
Make sure that under 'Info - Flags' 'External' is checked.
Set 'Info - Unit' to unit you want to follow host.
Exit out of 'Ability - Info +'
Under 'Ability - Leash' set the distance you want following units to be able to travel before returning to host.
Set 'Data - External Angle' to where you want units to be placed. They must be placed in a circle around the host. For example, 0, 90, 180, 270 would have 4 units placed at equal intervals around host.
Goto your host unit and goto 'Ability - Abilities +' and add your custom 'Carrier - Hangar' ability to your hero.

Now, in in my example ability I gave the ghost this ability twice, one handled the marines and one handled the robots, since I wanted them to be positioned in certain ways. What this means is that when you want to add more than one type of unit to a host, you can either add more units under 'ammo' or you can just create a new 'Carrier - Hangar' ability to handle the new unit. The latter method would allow you to dynamically add or remove 'party members' from a party I believe, so I would suggest using that method.

Also remember to go into each following units' data entry and make them unselectable.

[Ability] Hero Party