How can I check that when an address does the opt out, that tx is not in a group. Or in general how can I check if a tx is not in a group. I checked the length of the group with 0 but this seems to fail. Is it wrong?
Length of the group should be 1 not zero, as the transaction is in the group.
OK, thanks, so even if a single tranasaction is broadcasted, this is ever seen by the protocol as a group of 1 txs?
Yes, 1 single transaction = 1 group of 1 transaction.