This article explains whether this is required and provides a template for people to use
Some option agreements (including the standard Vestd option agreement), contain a clause that obliges an option holder to sign a Deed of Adherence to any existing Shareholder Agreement, if required, by the company before exercising their options.
This is to ensure that the new shareholder abides by any existing arrangements that other shareholders have agreed to.
In this case, it is important that the new shareholders has seen, read and fully understands any obligations contained in such a Shareholder Agreement.
Attached to this article is an example Deed of Adherence for companies and their option holders to use.