Posted By MT
Personally, the first think I'd do is telephone your insurers. I'm involved in organising a community fireworks display, and believe it or not, insurers are more eager to give insurance for a fireworks display than they are for a bonfire, hence we don't have a bonfire any more. It may be that your insurers won't be at all keen on the idea.

Posted By David Bruce
As well as insurance you will need to speak to the local authority as they may spoil the party by calling out the fire brigade. I know this happened to my dad who was burning leaves and general waste in the garden. If your company is near a residential area where smoke could travel and pose a nuisance, I think it will be a non starter.

Posted By Ken Taylor
The main considerations seem to be: insurance; fire risk assessment; and HSE and DTI guidance for firework displays and public events - including: planning, notifying police, fire brigade, neighbours, etc, health and safety plan and emergency action plan, staffing and their duties, parking arrangements,
site selection, staff training, preparation, security, fencing, content, size, construction, checking bonfire before ignition, safe ignition, first-aid provision, site condition, weather (particularly wind) conditions adequacy of illumination away from the bonfire, fire safety provision (eg extinguishers and fire-blanket in case of casualties), and closedown strategy (eg making sure the fire is out before leaving. 100 well-behaved responsible adults may not require any fencing of the bonfire whereas larger numbers or less reliable elements will need to be physically kept at a safe distance. Children, drunks, dogs and people bringing their own fireworks can all be trouble.

Posted By Paul Oliver
Also consider the make-up of your bonfire, as it is illegal to burn commercial waste.

Posted By mike morland
Just to add to Ken's comprehensive list - don't forget toilet facilities with consideration of attendants to keep floors clean (slips, trips etc.)