Most people inadvertently affect the reputations of groups they are seen as part of while they go about other activities. But some people also purposely exploit the fact that their behaviour and thoughts will be seen as evidence of those of a larger group, to give the false impression their views are widely supported. These people are basically stealing the good reputation of groups; they enjoy undeserved attention and leave the groups’ images polluted.
Such parasites often draw attention to what a very ordinary member of the targeted group they are, or just straight out claim to be speaking for that group. People who ‘have been a left voter for fifty years, but this year might just have to vote conservative’ are getting much of their force from implicitly claiming high representativeness of a large and respected group, and those who claim they write ‘what women really think‘ are more overt. From the perspective of women who think for instance, this is almost certain to be a damaging misrepresentation; any view other than your own is worse, and people who have good arguments are less likely to steal the authority of some unsuspecting demographic as support. It is also costly to listeners who are mislead, for instance about the extent to which women really think. Costs of prevention ignored then, less of this is better.
Purposeful exploitation of this sort should be easier than other externalities to groups’ reputations to punish and to want to punish; it’s easier to see, it’s directed at a specific group, and it’s more malevolent. However the public can’t punish or ignore all claims or implicit suggestions of representativeness, as there are also many useful and accurate ones. Often much of the interest in learning what specific strangers’ views are requires assuming that they are representative, and we keenly generalize this way. So mostly it is up to groups to identify and punish their own dishonest exploiters, usually via social pressure.
This means groups are easier to exploit if their members aren’t in a position to punish, because they don’t have the resources to deny respect that matters to the offenders. If you claim to be broadcasting what women think, most women don’t have the time or means to publicize the shamefulness of your malicious externalizing much. Even if they did they would not have much to gain from it personally, so there is a tragedy of the commons. And in big groups it is hard for a member or several to know whether another supposed group member is lying about the group’s average characteristics; they may just be a minority in the demographic themselves. Respectable groups are also good. Last, if most people have a lot of contact with the group in question, and the topic is a common one, it will be harder to misrepresent. So large, respectable, powerless or otherwise engaged groups who don’t commonly discuss the topic with the rest of society are best to make use of in this way.
I haven’t seen this kind of activity punished much, it doesn’t seem to be thought of as especially shameful. But given that, it seems rarer than I would guess. For instance, if you wanted to push a radical political agenda, why join the disrespected minor party who pushes that agenda rather than a moderate party, which allows you to suggest to your audience that even the larger and more reputable moderate party is coming around to the idea?
Meteuphoric 
“I’ve been an X voter for n years but will now have to vote Y” probably isn’t the best example of intentional defamation you describe in this post. Sure, there are notable concern trolls (and these are identified as useful with amusing effectiveness by various parties), but the statement itself doesn’t indicate a surreptitious attempt to speak on behalf of X voters. It is inferred as such by others. That statement could just as well describe party X’s (actual) fall from grace in the mind of that person.
The rest of the post is spot on. I’d like to add that sometimes, self-anointed spokespeople become anointed officially, by a majority of the group. The followers then associate everything that comes out of their chosen representative’s mouth with defining characteristics of whatever movement they happen to comprise. See the influence of bloviating talking-heads in American politics, or how “conservatism” has come to simultaneously espouse individual liberties and socially restrictive behaviour for certain groups (homosexual marriage, abortion, etc.).
The hostile-to-cryo wives (“what women really think”) article you linked is indeed damaging.
Maybe this is just sleep deprivation wrecking my verbal processing ability, but I’m wondering what this post’s title is supposed to mean.
As for radical political agendas and moderate parties, that’s more likely to harm the moderate party (the opposition will use the incident to portray the moderate party as extremist; this sort of thing happens all the time in American politics) than advance the agenda.
Perhaps a holdover from our evolutionary past? In small tribal groups rumors spread fast. It’s simply not worth the risk to your reputation. So despite today’s huge group sizes and urban anonymity we still act as if a stranger exposing us is the end of the world.