Can you get insurance without a vehicle?

yes. see previos answer.

If you do not own a vehicle you can still purchase auto insurance to cover you when you are driving borrowed vehicles and rented vehicles. Named non-owners insurance is a policy for individuals who...

Not in the UK.

Yes, In the United States you can purchase a Vehicle without having insurance on it beforehand. You just can't drive it off the lot or anywhere else.

Yes, if they have insurance and get into an accident, then their insurance will cover them. If they do not have insurance, then your insurance will cover them but your premiums may go up.