§1. The person presiding over a college or group is to convoke all those belonging to the college or group; the notice of convocation, however, when it must be personal, is valid if it is given in the place of domicile or quasi-domicile or in the place of residence.
§2. If anyone of those to be convoked was overlooked and for that reason was absent, the election is valid. Nevertheless, at the instance of that same person and when the oversight and absence have been proved, the election must be rescinded by the competent authority even if it has been confirmed, provided that it is evident juridically that recourse had been made at least within three days from the notice of the election.
§3. If more than one-third of the electors were overlooked, however, the election is null by the law itself unless all those overlooked were in fact present.
Source: Code of Canon Law (Vatican.va)