Insurance advice

We are just about to get our first motorhome - an Elddis Majestic 195 (2014) and would like to put other family members on the insurance as well as my wife and I as they will be making use of it from time to time . My daughters and partners are in there 30s. Any advice as to the best way, both cost wise and hassle free . Is it best to put them on annually or as and when they want to use it . Also recommendations would be great.

Many thanks

Derek