Apparently, Luke actually traveled with Paul on a few different occasions.
In much of his narrative, he chronicles the events that Paul and his companions were doing.
But on occasion, he changes the personal pronouns from *he* and *they* to *us* and *we*.
It’s kind of interesting reading that in Acts and noting when it happens.
Where do you find this?
I would say that's likely when Luke met the apostles and joined Christianity for the first time.