Justification of Offertories according to the Regulative Principle of Worship

I was in a church today that considers itself to hold strictly to the regulative principle of worship (that which is not explicitly commanded for public worship in Scripture is implicitly condemned) and was puzzled that a choir sang an offertory—sang without the congregation during the taking up of the offering. I am wondering, given their position on worship, what the Biblical justification for this practice might be. Is this practice defensible from Scripture, given the regulative principle?