Insurance is a must (liability issues), as is first aid training which has to have up to date certification.
Also, I think you'd need to evidence an appropriate level of risk assessment planning, appropriate to the area you're going into and the level of ability of the individuals you are guiding.
Plus remember that if you're going off public rights of way, then I believe there are issues for using Access Land (under the CROW Act)if you're getting paid by the party you're guiding.
I help out as an assistant ride leader for an equestrian company that takes/guides horse riders on the fells, and there's loads of red tape designed both to keep your 'customers' safe and you too (from being sued).