What's involved with file sharing with users,companies outside your company? DMZ? Certs? What else?

Does memory serve me right, that besides it's obvious internal use by your own company's users, that SharePoint can be for external collaboration with outside people/companies?

Is there a good MVP out there that has a good guide on setting this up?   I imagine your SP server would need to be in (or have a nic in) your DMZ, and you'd have to setup a real certificate in either IIS ? or Sharepoint? or both?   (we do have a GoDaddy certificate)

And then how to give external people access would be a whole other thing I imagine.

If this is possible, I'd need detailed steps on all of these, especially anything on certs.