Sioux Falls Federal Credit Union has been open since 1934. It's the 3rd largest credit union in South Dakota with assets totaling $297.7 Million and providing banking services to more than 30,000 members. Membership: The credit union is a community-based financial institution. People living, working, or worshipping in the credit union's geographic area can become a member and access banking services. Membership is also open to immediate family of current members.