How safe?

Hey Guys,

My blog has a few authors, and those authors are all my friends. I just got an email from someone asking to blog for WizzHack (my blog), so I was wonder, how safe is this? If I give him a login, and Author rights, then since he is in the back end of my website, is it possible to release some kind of MySQL script, or upload a file that could ‘hack’ my blog?

Thanks,

WizzKid