Use tokenGroups:
DirectorySearcher ds = new DirectorySearcher();
ds.Filter = String.Format("(&(objectClass=user)(sAMAccountName={0}))", username);
SearchResult sr = ds.FindOne();
DirectoryEntry user = sr.GetDirectoryEntry();
user.RefreshCache(new string[] { "tokenGroups" });
for (int i = 0; i < user.Properties["tokenGroups"].Count; i++) {
SecurityIdentifier sid = new SecurityIdentifier((byte[]) user.Properties["tokenGroups"][i], 0);
NTAccount nt = (NTAccount)sid.Translate(typeof(NTAccount));
//do something with the SID or name (nt.Value)
}
Note: this only gets security groups