This is our first big clue that the poem is dipping into societal issues. The Majority of whom? Society, that's whom. Before this point, the poem might only be about crazy vs. uncrazy for all we know. This is the place where the speaker shifts gears a bit, and lets us know a there's little more going on. Notice that this is also the only line with no dash after it—another hint that it deserves a little extra attention.
Assent—and you are sane— Demur—you're straightway dangerous— (6-7)
You could argue that these lines aren't really about people with mental disorders at all. Instead, they're about people with political ideas that go against the norm. For example, in Dickinson's day, it might be somebody saying, "Hey, how about we actually have equal rights for every person in this country. No, like for real. Everybody."
And handled with a Chain— (8)
Society always has penalties for folks who choose to go against the grain. The speaker uses the symbol of a chain, but you could substitute for a bunch of other stuff—prison, public ridicule, social isolation. When society doesn't want to hear the "complainers" out there, it's really good at finding ways of shutting them up.