How did plentyoffish.com get my name and work email?

A lot of sites rent mailing lists from other site operators, thus "passing around" known good addresses. Others just hire spam jockeys to do things as simple as just probing servers for "good" addresses randomly, pushing brute force string discovery for addresses, or even to exploit poorly configured mail servers, often in corporate settings, with various cc: and Bcc: "envelope" exploits.

Could a co-worker have signed you up as a joke?

I'm with oddman. I think someone signed you up as either a joke or to mess with you.

Is your name guessable from your email address? [email protected]?

spam operators brute-force email addresses, so never using or giving out an address doesn't stop brute-force spam coming in on it. Some of them have software that guesses your name from the address and incorperates it into the email, so if, as sohcahtoa asks, you name is guessable from the address, then the email might be nothing more than automated spam.

Well, look at it this way. Some website I've never heard of sends you an email. You say you've browsed this site from time to time. It's overwhelmingly likely that you simply gave your name and that email address to them sometime without realising it -- it'd be a pretty big coincidence otherwise, no?

I'm unfamiliar with the site -- is it the kind where you can input the contact info of the person you want to talk to, without knowing whether that person has an account or not? Like, maybe someone who knows you through work thought "Oh I want to talk to zorro astor, let me plug in his email and have the system invite him"?

plentyoffish is run by a single person- it's not a corporation with multiple layers that would go through the lengths to buy a spam list. Additionally, since it is totally free, and not subscription based, it doesn't make sense for it to actively recruit you. As such, it is probably a friend messing with you.