I'm not quite sure how Spell Defense works when more than one magician is protecting a group. SR 176 says "Handle it as Teamwork." The Teamwork rules are regarding tests though... Spell defense just adds dice.

What I think it means: Ace and Bob are both mages protecting the party from a Powerball. Ace has Counterspelling 5, Bob, Counterspelling 3. Ace is the primary one. So Bob rolls his Counterspelling dice, and for every hit he gets, one die is added to Ace's Spell Defense dice for everyone else?

Pretty much: it works like this:

The mage with the lower counterspelling rolls his counterspelling dice. For each hit he gets, the other mage gets an additional die to roll for his counterspelling test. Hits generated by the second mage count as resistance successes for the target(s) of the spell.

Note that counterpelling-for-resistance only gets rolled once per spell, regardless of the number of targets. (Your wording was slightly vague there.)

I'll take a stab at this too.

Let's say an enemy mage, Sue, is Bob with a F5 stun bolt. Both Ace and Bob have declared spell defense for Bob.

1. Sue rolls 11 dice for her Magic + Spellcasting + Focus, scoring 4 hits.
2. Bob rolls his 3 counterspelling dice, scoring 1 hit.
3. Bob then rolls his willpower of 5, plus Ace's 6 counterspelling dice (5,+1 for Bob's hit), scoring 2 and 2 hits respectively for a total of 4.
4. With no net hits, the defender wins and the spell fails

Now let's say the enemy mage is using a F5 stun ball. Both Ace and Bob have declared spell defense for Bob.

1. Sue rolls 11 dice for her Magic + Spellcasting + Focus, scoring 4 hits.
2. Bob rolls 3 dice, scoring 1 hit.
3. Bob rolls his 3 counterspelling dice, scoring 1 hit.
4. Bob then rolls his willpower of 5, plus Ace's 6 counterspelling dice (5,+1 for Bob's hit), scoring 1 and 2 hits respectively for a total of 3.
5. Ace rolls his willpower, 6 dice, and scores 2 hits, which added to the 2 counterspelling hits above total 4 hits.
6. Sue fails to effect Ace, but has hit Bob for 5S this time.

What becomes tricky is when area spells get involved and someone tries to spend edge. Are the counterspelling dice rerolled for everyone, or just the person who spent edge? Can anyone spend edge to reroll the counterspelling dice, or just the person who contributed the biggest counterspelling pool?

Eep!

Wow, Thanks Mr. KUQ(RNF #IQN. I usually miss the "Roll only once" part.
Teakwork Test, that I knew.

This can get really effective, if you have enough people with counterspell. You can soak pretty huge spells if you can routinely generate 10+ dice in counterspell.

I'd argue that the senior guy is the only one who needs to use edge, then he gets to reroll all the dice HE gets. So if Bob has counterspell 5 and gives his 2 hits to Dave to use with his counterspell 7 Dave can use 1 edge and reroll any of the 9 dice he has that miss. He can't reroll Bob's 3 dice that didn't get a hit. Bob could reroll his 3 with a point of edge.