Use the “+ add” menu in the navigation bar. If the artist is already on Bandcamp, choose “existing artist.” Requesting access from the artist will send them an email invite. If they accept, this will give the label the ability to add new releases to their page. Alternatively, adding the artist page via password authentication will give the label full access to their account. If the artist doesn't yet exist on Bandcamp, choose “new artist."
If you created an artist page, you can give the artist access to the page with a unique login that will be separate from your label login–just roll over their image on your Artists page, select “give access”, and enter their email address. This will let them sell their own items, edit the page design, and message their fans.
There's also a "remove" link should your relationship come to an end and you wish to transfer the page and its contents over to the artist. Removing an artist does not update any payment settings, but artists will be able to change the payment recipient on all items on their page once they're removed.