Can other characters join in on this attack?

Nope

The effect doesn't allow a standard attack phase, it create an attack with him as an attacker, skipping the phase where you choose all participants.

See FAQ on Quick Strike:

Q: Can characters with the ranged keyword participate in an attack declared through the card Quick Strike (CORE 35)?
A: No. There is no opportunity for other characters to join a Quick Strike attack. One character is exhausted to pay for the cost of Quick Strike, and the effect is that the exhausted character is immediately declared as an attacker against the target enemy. The card’s resolution does not allow for a standard declaration step in which other characters can declare.