You can do both..
If you have registered, and are logged in, then you should be able to add existing ones by clicking on a link when you view a release..
It says something obvious like "Add this Release to My Collection"
If you want to add a release thats a completely different kettle of fish, Ive
just tried adding a few and there is a specific format you have to follow..some
call it petty, but Im willing to stick with it
Its going to be useful because I really cant be arsed typing out all the records I have..
It details in the FAQ how to add a release but its not very intuitive unfortunately, but on the brighter side there are a couple of mods from Discogs on here who are helpful