When the legislative body adopts an ordinance declaring the need for an agency, the mayor or chairman of the board of supervisors, with the approval of the legislative body, shall appoint five persons, who are resident electors of the community, and may include tenants of a public housing authority created pursuant to Part 2 (commencing with Section 34200) of this division, as members of the agency. The legislative body may, either at the time of the adoption of the ordinance declaring the need for an agency or at any time thereafter, adopt an ordinance increasing to seven the number of members to be appointed to the agency. Upon the exercise of such option by the legislative body, the membership of the agency shall remain at seven.