As I understand it, you have to have third party insurance from a particular provider (CTC or BC) to sign up for a FNRttC. (More detail
https://www.cyclechat.net/threads/london-to-brighton-night-ride-march-6th.172399/post-3506168 ). Not that they could really stop you riding the public roads anyway.
IIRC, although FNRttC doesn't use this feature, CTC does allow the groups it insures to have a small number of non-members for a small number of rides but basically they want every rider to pay the £46 annual membership ASAP. I don't know BC's current requirements for groups and I've not looked recently because I don't like being disappointed!
I think insurance is much more important if yours is a formal ride with all the directing hand signals and shouts of "clear" and so on, or engaging in risky close-formation riding. If it's just a group of workmates and family riding along a consensual route along public roads and cycle tracks at the same time in a similar manner to any coincidental group of people (with due consideration to the Highway Code and so on), then everyone's ordinary third party cycling insurance (whether through household insurance, another club or a specialist scheme) should cover any incidents, shouldn't it?
Oh and besides directing people, the other big thing to beware is fixing other people's bikes. CTC group insurance does not cover it, but insurance has been offered free-of-charge during Bike Week by its organisers in recent years. Some workplace insurances cover it for work-associated groups - maybe more likely if a workplace has pool bikes or does similar light mechanical work? In general, it's probably best to advise people on how to fix their own bikes for other reasons too, though.