The guides have to have passed through the Baptismal Light first. Then they can petition the Heavenly Council to be a guide of a particular individual. They might pick someone out when they’re a baby; then they stay until you pass, or fall from grace, if that should happen. Yes, they volunteer. You can have more than one.
Does everyone have a guide?
Everyone has guides, unless they are deemed unworthy. That would be willful satanic followers, criminals, etc. Some people learn of their guide or guides at different times in their lives. But regardless of what period the guide or guides are assigned, all of us are being watched by Holy Ghosts at all times.
How long is your guide remain with you?
Unless you fall from grace, they stick with you until you pass over. Then they get you situated though the Light and on the other side. After that, they’re free to re-petition for a new human subject.
If that happens, can you get a new guide?
You can get a new guide, if you’ve fallen from grace, and later been given a reprieve for your sin. But some further atonement might be in order. It is up to the Creator in each individual situation. It chiefly depends on the sinful act or acts committed.