enable new user registrations after admin approval.

If you keep user information in database, you can have a column like UserStatus. In the code, it should be something like enumeration a bit set:

[System.Flags] enum UserStatus {
    None = 0,
    ApprovalPending = 1, //approved if this bit it clear
    Logged = 2,
    //... powers of 2
    Initial = ApprovalPending, // used only internally 
}

//usage:
UserStatus userStatus = UserStatus.Initial;
//same as UserStatus.ApprovalPending; //initial status immediately the user account is created

//to check a set element:
bool approvalPending == (userStatus & UserStatus.ApprovalPending) > 0;

//to include:
userStatus |= UserStatus.ApprovalPending;

//to exclude (approve):
userStatus ^= UserStatus.ApprovalPending;

Your database can store one numeric value in this column, a bitwise combination of UserStatus enumeration values, you can cast it from numeric type to UserStatus and back.

For approvals and other administrative actions, you can have a special administration Web page, say, password protected, or devise a special database access application accessible only within the local network.

—SA