The ActiveDirectoryService class in its original state, before refactoring.

ActiveDirectoryService - Original

public class ActiveDirectoryService
{
    private const string GroupNotFoundMessage = "The directory group, '{0}', could not be found.";
    private const string UserNotFoundMessage = "The user account, '{0}', could not be found ";

    private readonly string _adDomain;

    public ActiveDirectoryService(string domain)
    {
        _adDomain = domain;
    }

    public bool IsMember(string accountName, string groupName)
    {
        using (var context = new PrincipalContext(ContextType.Domain, _adDomain))
        using (var group = GroupPrincipal.FindByIdentity(context, IdentityType.SamAccountName, groupName))
        using (var user = UserPrincipal.FindByIdentity(context, IdentityType.SamAccountName, accountName))
        {
            if (group == null)
                throw new NoMatchingPrincipalException(string.Format(GroupNotFoundMessage, groupName));

            if (user == null)
                throw new NoMatchingPrincipalException(string.Format(UserNotFoundMessage, accountName));

            return user.IsMemberOf(group);
        }
    }

    public void AddUserToGroup(string accountName, string groupName)
    {
        using (var context = new PrincipalContext(ContextType.Domain, _adDomain))
        using (var group = GroupPrincipal.FindByIdentity(context, IdentityType.SamAccountName, groupName))
        using (var user = UserPrincipal.FindByIdentity(context, IdentityType.SamAccountName, accountName))
        {
            if (group == null)
                throw new NoMatchingPrincipalException(string.Format(GroupNotFoundMessage, groupName));

            if (user == null)
                throw new NoMatchingPrincipalException(string.Format(UserNotFoundMessage, accountName));

            group.Members.Add(user);
            group.Save();
        }
    }

    public void RemoveUserFromGroup(string accountName, string groupName)
    {
        using (var context = new PrincipalContext(ContextType.Domain, _adDomain))
        using (var group = GroupPrincipal.FindByIdentity(context, IdentityType.SamAccountName, groupName))
        using (var user = UserPrincipal.FindByIdentity(context, IdentityType.SamAccountName, accountName))
        {
            if (group == null)
                throw new NoMatchingPrincipalException(string.Format(GroupNotFoundMessage, groupName));

            if (user == null)
                throw new NoMatchingPrincipalException(string.Format(UserNotFoundMessage, accountName));

            group.Members.Remove(user);
            group.Save();
        }
    }
}