It depends on their nationality. If they can obtain a visit visa on arrival they can just fly over and pay a small fee (it was QR100 as of a few months ago) otherwise they need to get a visa in advance. If that is the case I would contact your embassy here to get the required steps.

Here is a link to the 33 nationalities that can get a visit visa on arrival.

http://english.mofa.gov.qa/visa.htm