Who made up the First Fruits resurrection?

The First Fruits resurrection was comprised of Christ when He rose from the dead, along with the Jewish believers who came out of their graves in Matthew 27. These together made up the First Fruits, as distinct from later resurrections.

Les teaches that in 1 Corinthians 15:20 Paul shifts gears to explain the order of resurrections. The First Fruits took place when Christ rose from the dead, and it also included those Jewish believers who came out of the graves in Matthew 27. Les says these together comprised the First Fruits. Then, as Paul states in verse 23, every man is in his own order: first Christ, the First Fruits, and afterward those who are Christ's at His coming, which would be the believers of the church age. Les emphasizes that the resurrections are not all at one event but occur in sequence.

Matthew 27:52

And the graves were opened; and many bodies of the saints which slept arose,

Matthew 27:53

And came out of the graves after his resurrection, and went into the holy city, and appeared unto many.

Are all the resurrections going to happen at one event, and who comprised the First Fruits?

No, they are not all at one event. Paul sets an order: first Christ, the First Fruits, and afterwards those who are Christ's at His coming. The First Fruits comprised Christ when He rose and the Jewish believers who came out of the graves in Matthew 27.