Different business structures need different contracts for the owners.
In the UK there are three main business structures: sole trader, partnership, limited company. ContractStore's Guide to Starting a Business, document Z160, is a free document explaining the different business structures.
If you are setting up a limited company we have shareholders' agreements . A shareholders agreement sets out the arrangements for owning shares and for managing the company. ContractStore has three such documents: Heads of Agreement (document A108) is a short form for three people setting up a new company. A107 is for two shareholders, A166 for three or more. Use a Deed of Adherence, A193, when a new shareholder becomes a party to a shareholders agreement.
If you have a partnership, we have Partnership Agreement templates, for 2 partners: document A148; for 3 partners or more, A149. Our Limited Liability Partnership Agreement is A152. A LLP is in some ways a cross between an unincorporated partnership and a limited company. The LLC members have limited liability.
You can download free ContractStore checklists for the different business structures. Shareholders Agreement Checklist is Z142, Checklist for Joint Venture Z145. We also have free guidance on Preparing a Business Plan Z155.