Hi Michael
A few disjointed thoughts.
I think you need to be clear what is the main reason behind talk of forming a committee. Is it to essentially take over the management of the site on some level because tenants are unhappy at the conditions placed on their leases by the owner?
If so, unless the site owner agrees to the committee being involved in self-management (on whatever scale) then it's a non-starter in my view. It's his land and he can use it in whichever way and subject to whichever restrictions he chooses. If tenants are unhappy then they have the choice to leave. (This is not intended to be harsh but it's the reality of private allotments).
If the site owner is happy to allow tenants to set their own restrictions on the land, I'd be very cautious. There have been a number of posts here in the past from tenants, often though not exclusively on smaller private sites, where the committee has become a powerful clique and there's no challenging it. Particularly where there's less protection against being thrown off your plot if you raise your head above the parapet.
If the owner is wanting to completely relinquish his role in running the site (i.e. full self-management) then that brings its own challenges.
You'd need to set up an association, draft a constitution providing for quorum, meetings, election of officials, powers to bank etc. Also what the procedure for warning letters will be, who decides upon eviction, is there a right of appeal. Who will be responsible for site maintenance? What about issuing invoices, collecting rent, holding the money - setting up a bank account.
Having control over one's activities can be very rewarding but it takes a lot of time and hard work, and that depends on constantly having members prepared to step up and do the work. I've seen more than one site get into disrepair when its management committee became a bunch of disorganised egos and nepotism. That site is now thriving, it must be said, but there are a few people who put in 10's and 10's of hours down there per week in order to do it well.
Good luck and come back if you've any more questions