“For where two or three are gathered together in my name, I am there among them.” (Matthew 18:20)
Read in light of 18:15-19: Christ stands in unity with the gathered, recognized, identifiable church as it acts in agreement in receiving or removing believers from fellowship, after practicing due diligence through gradual escalation and protocol. Even if the church is two people big.
This does not mean that a church (acting properly?) can be a mere ad hoc or irregular or accidental gathering of believers, let alone one that has no regard for protocol, due diligence, measures of escalation, or awareness of membership.