Tourist visa (LOI issue) (Lagos)

Hi all.

I'm currently working in Lagos on a standard business visa.
I plan to invite my Scandinavian girlfriend to come visit. Can anyone help my concerning the requirements for the "Letter of invitation" for a tourist visa? I get different information depending on where I ask. Can I invite? Does it have to be a Nigerian resident or even local company inviting?

Thanks a bunch for your advise.