Staying Over night in Mount Koya

So basically, since I am travelling with people, some wanted to try staying overnight at a temple whilst doing Mount Koya's Hiking trial in Kansai.

Has anyone ever stayed overnight at a temple, and how do I go about doing this or making reservations?

http://www.japan-guide.com/e/e4905.html look at the links too

https://www.japaneseguesthouses.com/ryokan-single/?ryokan=Henjoko-in

I found this. Does this mean that I can stay there but there will be no one there at all?

Depending on when you're looking, there may be no availability (e.g. Golden Week - May 4th to 7th) - so I would advise booking asap if you're looking at a date around that time. As an FYI - while looking at several places over 2 months ago, I was only able to find availability at a few.

As mentioned above, check out the resources at the bottom of the page above. I found the individual sites (listing here - http://eng.shukubo.net/temple-lodging.html) more helpful than Japanican, etc.

Good luck!