UserPrincipal user = UserPrincipal.FindByIdentity(new PrincipalContext(ContextType.Domain, "<Domain>"), IdentityType.SamAccountName, "<UserName>");
foreach (GroupPrincipal group in user.GetGroups())
{
    Console.Out.WriteLine(group);
}

String domainName = @"<your domain>";
        String username = domainName + @"<your username>";
        PrincipalContext thisDomain = new PrincipalContext(ContextType.Domain);

        UserPrincipal userPrincipal = UserPrincipal.FindByIdentity(thisDomain, username);

        PrincipalSearchResult<Principal> userGroups = userPrincipal.GetAuthorizationGroups();

        foreach (Principal principal in userGroups.OfType<GroupPrincipal>())
        {
            Debug.WriteLine(principal.Name);
        }