public static bool IsUserMemberOf(string sGroupName)
{
    using (PrincipalContext ctx = new PrincipalContext(ContextType.Domain))
    {
        using (GroupPrincipal groupPrincipal = GroupPrincipal.FindByIdentity(ctx, IdentityType.Name, sGroupName))
        {
            if (groupPrincipal != null)
            {
                using (UserPrincipal userPrincipal = UserPrincipal.Current)
                {
                    if (userPrincipal != null)
                    {
                        return userPrincipal.IsMemberOf(groupPrincipal);
                    }
                }
            }
        }
    }
    return false;
}

return userPrincipal.IsMemberOf(groupPrincipal);