“Employee”: Any person in the service of a board of park commissioners as described in Section 12-101, or employees of the board, except (a) any person employed in any position requiring service of less than 90 hours during a monthly period, or, beginning July 1, 1991, in any position requiring service of less than 6 months during a calendar year, unless that person is already a member of the fund, and (b) any employee of a city or district transferred to the employment of a park district by virtue of the Exchange of Functions Act of 1957.