You can become a shareholder in a SAS company either as an individual or as a company. In either case you can be an investor, a legal representative (CEO) or both!
The difference resides in the liability you are willing to assume, personally or through a company.
As an individual, your name will appear in the bylaws of the new company as partner and if you are also CEO, you're liable as an individual (within the limits of your investment, see section liability of shareholders).
Through a company, this company is the shareholder and the signatory of the documents must have a power of attorney. If this company is also a CEO of the new company, it will have to designate an individual as representative, but the limited liability will be assumed by the company.