They're very simple. If you know an user on the site you've done work for, you can click the image you've made for them, then enter their name in the space to send a collection offer.

Then if they accept the collection, it will show up in the notification list of people who watches them as them having "collected" the work and will link back to the version you posted if they click the thumbnail there :)

Some artists go the other way about things, letting their commissioners instead post the ar themselvest and basically just sit by and wait for the collection offer from the commissioner(which for the artist will still show up in the notification list of their own watchers as having "collected" said work from the commissioner... and thus a way to advertise to their watchers they do take on commission for others without going through the hassle of posting all the artwork themselves on their gallery).

I'm gonna start leaving requests ONE EVERY SINGLE THING YOU DREW.

Until the system starts allowing users to extend requests to artists, I don't think I can be bothered. I wouldn't expect you to track down the Weasyl accounts for everyone who's ever commissioned you, and I think an art site built on that premise is totally short-sighted :P