As long as any member of a Brood Brother unit is within 12" to either the Magus or the Patriarch, they count as having Leadership 10 for all Morale checks and tests to Regroup after Falling Back. This counts for Heavy Weapon teams as well as regular units, but can be modified as normal (outnumbered, below 50%, etc). These units can also test to Regroup even if they are below 50% of starting strength or within 6" of an enemy unit, but again only if they are within 12" of the Magus or Patriarch. Note this does not effect any other Leadership-based tests such as Pinning checks.
Important Note: Genestealer Cults need not be lead into battle by the Magus or Patriarch. In the case of neither of them in play, all of the above rules would be ignored. Hierarchs will not give any of the above special benefits, nor will their deaths require any special tests to be made.