In these examples, we are emphasising the possibility that they have been to France, for example in response to a suggestion or assumption that they have not. We can change the position of "not" to make different emphases: We can also use "might not have" to form questions. In the second sentence, "not" is used to emphasise the negative of the main verb in this case, "might have" describes a positive possibility that he did not send the invitations.