Units chosen from the same faction (that is, if they are from the same Warhammer Armies book or Expansion to the same Warhammer Armies book) are considered to be Bound by Blood, and are treated as 'friendly units' from all points of view.
This means that, to give some examples, Bound by Blood units:
Can be joined by allied characters,
Can use an allied General's Inspiring Presence rule.
Can use an allied battle standard's Hold Your Ground! rule.
Cause Panic tests in allies when they are destroyed, flee through allies, and so on.
Are counted as being 'friendly' units from the point of view of targeting spells, abilities and so on. E.g. they can be targeted by augment spells, but not hex, direct damage or magic missile spells, and so on.