Ask HN: Best Practices for asking co-workers to help with your side project?

Hi, this is a throw away account if you haven't guessed already.

I work with some amazingly talented people, whether they be developers, designers or marketing experts, and thought it would be great to work with them on my side project. My question is, what's the best way to approach them and to make sure that the company we both work for understands that this is simply a client/contractor relationship and would not interfere with the work we're both hired to do?

Just note, I'm not trying to hire them away from the company nor am I trying to get them to work for free as a favor to me or for some equity. I think it's just a faster way to get the work done I'd already know them. No browsing through elance or odesk or interfacing with someone far, far away.

Thanks for your input!