Credit unions organized and existing under the Credit Union Act may organize and have membership in a central credit union to which federal credit unions organized and operating in this state may belong and in which officials of both such credit unions may have membership. Organizations which are organized for the purpose of furthering credit union activities and their employees may have membership in such credit union. Small employee groups of fifty or more employees having a common bond of occupation whose probability of a successful operation would be limited because of the lack of adequate membership may join as a group in the central credit union and become members of that credit union with all the rights existing under the act.