OMG this is doing my head in – help please

Can anyone tell me in simple English how a new user logs into WP?

I’ve created a new user who I want to write posts on my blog.

Problem is, they can’t login. I assume they login through admin (though nothing ANYWHERE in WP tells you this is what happens – so I could be wrong?). They just get “invalid username”.

Is there some sort of validation email they’re supposed to get? And how would anyone know? There’s nothing I can find that explains what is meant to happen next. Doh.

This seems like it should be so obvious, but I’m utterly baffled. Help please, if anyone’s done this. Thanks!