Being with Christ since John's baptism, was a requirement for being one of the 12, not for being an apostle. Otherwise Paul, Barnabas, etc. would have been disqualified.

Barnabas was an apostle. Acts 4:4, 14. Timothy and Silas--I Thes. 1:1, 2:6. Possibly Apollos. I Cor. 4. Possibly Andronichus and Junia--Romans 16.