If you are referring to adding them as a FAVORITE ARTISTS (not Friend as that is different) then there are a couple of ways.

First if you know a specific artists username and you want to add them as one of your Favorite artists, then  go to the GALLERY link on the Top navigation menu bar. This takes you to the main gallery page. From there you will see several links

[what's new] [most viewed] [most commented] [best ratings] [by artist] [by vendor]

Click the "by artist" link and type in the username of the artist.  this will take you to the full gallery of that artist. You can then click on any thumbnail in his/her gallery and it will bring up the FULL IMAGE and underneath  the full image you will see several buttons.

zoom in add image add artist

To add the artist, you click the ADD ARTIST button (as shown above).

Now if you don't know the artist username or you do not have one specifically in mind. As you browse the galleries anytime you click on a thumbnail those same buttons would be there and you can use the "add artist"

If you are referrring to the Friendslist (this is for members that you are online friends with, you will NOT get an email when they upload a new image like you do with the Favorite Artist add). you will need to get to the artist homepage first of all and you can do this various ways. You can click the HOMEPAGE link beside their avatar while you are browsing the gallery, you can click the homepage link underneath the avatar in a forum post., you can click the homepage link next to the avatar on the comments.  Once you get to that friends homepage, right beside their username on the first page it says "ADD TO FRIENDS", click that then they will get sent a sitemail so they can approve or deny this friends request.

Thanks