Hi my friend! As per my knowledge, travel agencies in Doha, Qatar can issue a hotel visa as a tourist visa. The rate will vary from the hotel. Your friends might need to actually stay in that hotel (minimum 2-3 nights stay, depending on which hotel). For a 3 star hotel, I think they are charging QAR 400-600 per passport for the visa(irregardless of age) plus QAR 300-400 range per night for the hotel. I suggest you visit or call any travel agency there. You might want to check from Qatar Embassy, but for sure they will ask you to do the same as I told you. =) If you call the travel agency the first time, tell them about the situation. Be ready to give some information about your friends in Riyadh, like nationality, date of birth and if they are all holding a residence permit in Riyadh, also their visa status and its validity. From there, the agent will tell you the next step. Goodluck!
Ciao.