This sample shows how to get group membership, and add a user to a group:

public static void LFDSMembershipSample()
{
    Server lmoServ = Server.Connect(lfdsServer, false);
    Database lmoDB = lmoServ.GetDatabaseByRealmName(lfdsDatabase);

    lmoDB.Login(new SessionTokenParameters() { });

    // Get all LFDS users
    List<User> allLFDSUsers = lmoDB.GetAllUsersEx(0, (int)UserFlags.ApplicationAccount);

    // Get all LFDS groups
    List<Group> allLFDSGroups = lmoDB.GetAllGroups();

    // Cache the mapping from SID to trustee
    Dictionary<string, ITrustee> sidCache = new Dictionary<string, ITrustee>();
    allLFDSUsers.ForEach(u => sidCache.Add(u.SIDString, u));
    allLFDSGroups.ForEach(g => sidCache.Add(g.SIDString, g));

    // Get all members for each group
    Dictionary<string, List<ITrustee>> allGroupMembers = new Dictionary<string, List<ITrustee>>();
    Dictionary<string, List<Group>> allUserGroups = new Dictionary<string, List<Group>>();
    foreach (Group group in allLFDSGroups)
    {
        var groupMembers = group.GetMembers();

        // allGroupMembers is groupSID -> <List of accounts in that group>
        allGroupMembers.Add(group.SIDString, groupMembers.Select(kvp => sidCache.ContainsKey(kvp.Key) ? sidCache[kvp.Key] : null).ToList());

        // allUserGroups is userSID -> <List of groups this user belongs to>
        foreach (var trusteeSID in groupMembers.Keys)
        {
            if (!allUserGroups.ContainsKey(trusteeSID))
                allUserGroups[trusteeSID] = new List<Group>();

            allUserGroups[trusteeSID].Add(group);
        }
    }

    // Report the groups that each user belongs to
    foreach (User user in allLFDSUsers)
    {
        Console.WriteLine($"{user.Name} belongs to :");

        if (!allUserGroups.ContainsKey(user.SIDString))
        {
            Console.WriteLine("(none)");
            continue;
        }

        foreach (var group in allUserGroups[user.SIDString])
        {
            Console.WriteLine($"{group.Name}");
        }

        // NOTE: Alternatively, you could call user.GetLFDSGroupMembership
    }

    // Assign a user to a group
    Group sampleGroup = allLFDSGroups.FirstOrDefault();
    User sampleUser = allLFDSUsers.FirstOrDefault();

    if (sampleGroup != null && sampleUser != null)
    {
        Dictionary<string, string> currentMembers = sampleGroup.GetMembers();
        List<string> newMembers = currentMembers.Keys.ToList();

        Console.WriteLine($"Adding {sampleUser.Name} to {sampleGroup.Name}");
        newMembers.Add(sampleUser.SIDString);

        sampleGroup.SetMembers(newMembers);
    }
}