Singularitarians are, IMHO, a subtype of Tranhumanists who are (1) focused on the Intelligence Explosion rather than nanotech, life extension, cryopreservation, Cyborg art, or any other Transhumanist area, and (2) do something about it.

Of course, people canbe Singularitarians and also other things as well, and people can believe something is a good idea without doing anything about it, but the above definition seems significant enough to be worth having a name for.

You could also potentially be a Singularitarian but not a Transhumanist, if you focused on AGI safety and even Friendliness, without caring about the entire H+ memeplex. Though there may be nobody like this today, I can imagine a future in which a game theorist or other scientist enters the field out of a combined desire to do good and to boost their career, but is not interested in all the other stuff.