Anyone who is with the person named on the registration can use digital kan-i radios. It is legal to lend/rent your radio to another person; this means that if you buy a set of radios you can hand them out to your friends to use for the day. If they are operating under your supervision such as skiing in your group (or are members of the registered owning organization) there is no need to file any more paperwork. If they are operating independently of you then you’ll need to let the prefectural authority know about it. See more details here.