First you need to carefully read the bylaws of this non-profit tax exempt corporation that have presumably been approved by the organizer of the corporation and other initial members in that its terms govern the protocol of how the corporation is run and how changes to the bylaws are made (notice requirements, number of votes required and the like).If the thought is to have non-voting membership in the corporation, one has to think about the purpose of this, set forth a plan to implement such membership, hold a meeting to discuss such before the corporation's current board of dircetors who will then make a recommendation. If the recommendation is approved for non-voting membership to be added to the corporation's bylaws, then notice of the proposal is sent to all voting members for a meeting to vote either in person or by proxy.The meeting is then held. If the requisite number of votes per the corporation's existing bylaws approve the request, the secretary memorializes such in the minutes and an addendum to the bylaws is created, signed by all voting members and placed in the books of the corporation with the corporate seal upon it.Good luck.