It is good practice to have a contract between shareholders that records the basis upon which they own and control the company. This is a private, confidential document that can deal with any number of issues, such as the transfer of shares and whether these should first be offered to existing shareholders, or where the shareholder is also an employee, what restrictions should apply if he were to leave the company.
This can be avoid costly litigation and acrimony in the future and is likely to be a small but very wise investment at the outset of a business relationship.