Paul is not referring to John’s brother James, who, being one of the twelve
(1 Corinthians 15:5), died a martyr’s death around fourteen years before the writing of this letter (Acts 12:1–2). He is referring to James the brother of Jesus. It is only after Jesus’ resurrection that James became a believer, as well as a great leader of the church (see John 7:5; Acts 1:14; Acts 12:17; Acts 21:18–19; Galatians 1:19; Galatians 2:9).
It may have been Jesus’ appearance to him that brought James to faith. Paul singles out James for the same reason he has singled out Cephas: here is a prominent and trusted leader of the church who can testify to the resurrection.
The fifth group is all the apostles.
Paul may have in mind a broader group of believers who saw the resurrected Jesus and served him by proclaiming the gospel (see Galatians 1:19; Romans 16:7). Another possibility is that he is referring again to the twelve
(1 Corinthians 15:5), who saw Jesus one last time before his ascension (Acts 1:6–11).
7 Then he appeared to James, then to all the apostles.