I was asked about this last night and surfed to no avail until I uncovered this. It's roughly the answer I gave.
"Lord's Supper in a small group setting?"
Throughout my Christian life I have always been uncomfortable when the Lord's Supper is served outside of the church, say, in a small group setting. I didn't know why until I dealt with the issue further in my RTS studies. Here's why such a practice should be discouraged:
Christ instituted the sacrament of the Lord’s Supper as a sign and seal of the new covenant He established with His church. The church has certain characteristics not found in small group setting. One of the most important is the oversight and rule of the elders. In a small group setting, since there are no elders present, it would be improper to administer the Lord’s Supper because the Lord’s Supper is a method of disciplining the congregation (withholding the Supper is the penultimate step of discipline - excommunication is the ultimate). Serving the Supper apart from the elders severs oneself from the sphere of discipline of the elders.
In summary, the administration of the Supper:
(i) is in the hands of the leadership of the church and,
(ii) accompanies the word, so it is to be administered by a minister.
While administering the Supper in a small group setting would indeed be an "irregular" celebration of the sacrament it would not be an "invalid" one. It would be impossible to argue that in celebrating the Supper in such a setting that there is no communion with Christ, however, it is clearly irregular and should be discouraged.
The same argument holds true for family communion, wedding communion, and church plant communion where the plant does not yet have elders."
My hunch is to say invalid, but it's based on the same argumentation regarding authority and discipline. Those two words are hardly popular in pop evangelical spirituality. The invidual is seen as authoritative in a twisted reading of the priesthood of all believers, and discipline is a bad word!
Barna's recent book Revolution seeks to applaud the entrepreneurial vein of pomo Christianity. He bids good riddance to the church and institutional authority in favour of sacramental understandings of Starbucks and any gathering between two Christians. This means that we need to redouble our efforts and retreat to the Scriptures to "sell" this notion of communion with Christ with his body in covenantal worship not just Christian fellowship.
Let me know if you can improve on these answers anybody.