When issuing or transferring shares to new or existing shareholders after you've incorporated, it's important that you review existing corporate documents to ensure that no existing shareholder has a prior right to purchase the shares you are issuing or transferring. 

The documents that you should be reviewing include:

Provisions in your Shareholder Agreement will override any restrictions that exist in your Bylaws or Articles of Incorporation. 

If you issue or transfer shares to someone not in accordance with existing corporate documents, then the share issuance or share transfer may become null and void.