Resistance buffs were simply added just as defense buffs were. That is, when a player (A) would get a resistance buff from an ally (B), the total resistance would be the sum of A's resists + the resistance buffs from B.
To further elaborate, let's say
A is any AT with 20% Defense to S/L, 15% Def to Melee, 10% Defense to everything else. 10% Resistance to all damage types.
B is a Cold/ Defender with no Defense or Resistance enhancements slotted in either Ice Shield or Glacial Shield
When B uses Ice Shield and Glacial Shield on A, this results in (numbers is parenthesis represent A before buffs + buffs from B, respectively)
Defense
Smash/Lethal 35% (20 + 15)
Fire/Cold 10% (10 + 0)
Energy/Negative 25% (10 + 15)
Psi: 10% (10 + 0)
Melee 30% (15 + 15)
Range/AOE 25% (10 + 15)
Resistance
S/L/E/N/P/T 10% (10 + 0)
Fire 20% (10 + 10)
Cold 35% (10 + 25)
Note that Cold shields only provide Resistance only to Fire and Cold, and only somewhat meaningfully to Cold type. Not that it's useless, that means the unique 2 Resistance IOs for 3% Def each can be slotted in those shields, for example.