A long time ago I was into what I called "aggressive content syndication" and I wrote a script that (over the course of two months) made several thousand users on an active developer's forum that didn't require email verification to make an account.

I made a lot of effort to make believable profiles for the users, they had first and last names randomly chosen out of a database and profile pics too, though I made no effort to match the pics with the name of the user (e.g. nationality, gender, etc.)

I had the users randomly upvote stories so if you looked at the upvote profile of the users it would look pretty normal. However when I finished a blog post I would have the system choose maybe 20-50 users to upvote my post and my post would go right to the top and usually get a large number of what I called "volunteer" upvotes.

I lost the database that had the users in a hard drive crash so that was the end of that project.

Note adding users at a high rate (usually many per second) is one of the more efficient ways to crash a web site because the users table in the database is frequently very active.

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Generally people like to spam links into forums and any place where it is possible to insert links and personally I don't believe it matters much if the links are "nofollow" or not.

Note it is more of a hassle to do this on a site that supports email verification, I used to set up highly interactive qmail servers that could do things like that but all the sign up emails would be on a limited number of domains that would stick out. I think the pro spammers have methods of creating large numbers of accounts at places like Google and Yahoo which are a great choice if you don't want your email addresses to stick out too much.

their trying to flag your domain as a spam source...once your emails are received they flag them as spam...with enough of those your domain will be in the black list