I get that the general point of it is that it's a party that focuses on Radiant damage and is apparently supposed to have a pretty high damage output, but which elements are needed to make it really powerful?
Step 1: Inflict radiant vulnerability
There's no point in everyone focusing on radiant damage if enemies aren't vulnerable to radiant damage. So make them vulnerable. The Morninglord paragon path from the Forgotten Realms Player's Guide is the preferred choice for this (its level 16 ability gives any foe you hit with a radiant power vulnerability 10 to radiant damage until the end of your next turn). There are a variety of other options, though, including some paragon-tier items and epic-tier feats.
Step 2: Inflict radiant damage
Once the target is vulnerable to radiant damage, you hit it with as much radiant damage as possible. Multi-hit powers are preferred, since each hit benefits from the vulnerability. Playing a class that doesn't have radiant powers? No problem! There are several weapon enchantments that can convert your damage to radiant.
Step 2A: Or just deal any kind of damage
The Pervasive Light feat from Divine Power gives you bonus damage equal to a target's radiant vulnerability whenever you hit with an attack that doesn't deal radiant damage. It requires you to have a divine class but multi-classing can solve that problem for most characters.
(Note that most of the tricks that make Radiant Mafia work so well don't kick in until paragon tier; a heroic-tier Radiant Mafia party will play just like any other party, other than maybe being a bit heavier on divine characters.)