Paul sent greetings in this verse to the believers who met in Philemon’s house. Groups of believers in New Testament times did not have church buildings, so they usually met in private homes.
Priscilla and Aquila apparently made their home available as meeting places in both Rome (Romans 16:3-5) and Ephesus (1 Corinthians 16:19). Early believers also met in the home of Nymphas (Colossians 4:15-16).
PHILEMON 1:2-3 To the church in thy [Philemon’s] house: Grace to you, and peace.