Anyone can sign it. I mean, anyone - your missus could sign it. Ideally an employer to say they're employing you as an electrician, but you can get anyone to sign it. It's just to say that you're working as an electrician; self-employed people on here have signed their own in the past.
If you do it on line they just email your 'sponsor' a link and they click on it. That's as far as it goes, there's no need to over-think it. I've asked supervisors on agency jobs to sign the sponsor section before - they're obviously convinced that I'm an electrician otherwise I wouldn't be working there, but they've flatly refused to do it because the JIB have worded it as if it means something official, like they're signing their life away.
It's a bit like the NVQ3 - just another hoop they make you jump through.