No matter how experienced you and your employees are, you cannot expect to have all the skills and knowledge your business needs, particularly as requirements change as your business grows. Growing businesses continually face new problems and opportunities. Advisors can be a good solution to get the advice your business needs. It is important to manage your relationships with them.
Non-executive directors are appointed using a Non-executive Director's Letter of Appointment (LOA). Non-executive directors do not run the business day to day. They normally work only part time but provide expertise that deepens the experience of your business and can make you more attractive to investors.
It's often mutually beneficial to compensate an advisor with shares in your company, and this can be agreed with a Founder Advisor Standard Template (FAST). This rewards the advisor with a small amount of equity if they reach certain targets. You could also reward a consultant through a Share Vesting Agreement.