Decide on the legal form you want your group to take: Will it be a partnership, company, society / club etc. The legal requirements will vary depending on what it is you are registering.
Visit the website of the Kenya Association of Investment Groups and download "what you need to know about investment groups" plus other stuff. It will help.
http://www.kaig.org/index.php Remember, each business form has its pros and cons. The company for instance has the most stringent compliance requirements (Taxation - 30%, Financial Reporting etc). You could start from a more friendly form (e.g. Society) and "graduate" into a company once you're sure what you're about.