The "social" Zombie feature causes the aggravated Zombie to summon Zombies, but the summoned Zombies appear to be able to spawn in otherwise un-spawnable locations, including within the player's no-spawn range, on non-spawnable blocks (half-slabs, stairs), and in areas with light levels above 7.

I have seen multiple instances of Zombies spawning within 1-2 blocks of a player (including literally in the same space as the aggravated Zombie).

The "social" Zombie feature should not enable Zombies to summon other Zombies in otherwise non-spawnable locations.