How to: Spawn an Army
Code:
script
spawn_army
faction romans_julii
character Flavius Julius, general, command 0, influence 0, management 0, subterfuge 0, age 20, x 89, y 80
unit roman generals guard cavalry, soldiers 20 exp 9 armour 1 weapon_lvl 0
unit roman legionary first cohort ii, soldiers 40 exp 0 armour 0 weapon_lvl 0
unit roman legionary cohort ii, soldiers 60 exp 0 armour 0 weapon_lvl 0
unit roman praetorian cohort i, soldiers 60 exp 0 armour 0 weapon_lvl 0
end
end_script
If you are playing as the Julii this will create an army banner with the relevant captain/family member as you define. Most of the syntax is the same as in descr_strat, however the missing clue was the positioning of the commas after the unit names as discovered here

Family members who are spawned, however, do not appear to join that faction's family tree.