Sentinel Federal Credit Union has been open since 1955. It's the 8th largest credit union in South Dakota with assets totaling $108.15 Million and providing banking services to more than 10,000 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.