As part of his exhortation to the participants, the Lord urges them to admit even "unworthy" people to their churches and synagogues, and to minister unto them. They are not, however, to drink the Lord's blood and eat his flesh, for fear of damnation.
I don't think this means that someone who wandered in off the street during an LDS sacrament meeting and ate the bread would be damned. I think this is an instruction to the same point as what is described in Alma 6: everyone was allowed into the general assembly, but the "children of God" met privately. The feast of bread and wine was a temple feast, and not everyone could participate.