If someone goes to a simcha (wedding for example) with in mind not to join together and therefore will not by chiyuv in a zimmun what if the person ends up staying until the end and now he is needed in order to make the zimmun may he join together (even though his das to begin with was that he will not join together with them.)
He can join with the zimmun. As with several laws of zimmun, it is possible to detach the obligation of zimmun from the capacity to join in.
For joining in, the fact that the meal was eaten together is certainly enough, and it is no worse than somebody who “joins for a zimun” by eating something (even fruit) at the end of the meal (no obligation, but joins for zimmun).
For further details, please see our articles on zimmun, where we have detailed the laws involved.