A Jewish wedding ceremony does not require a rabbi, nor a cantor, not even family or friends. It does require two witnesses, since it is a legal transaction. These witnesses must be able to testify in a Jewish court (beit din) the marital status of the couple, and they actually effect the marriage. Thus, this is a great responsibility.
Witnesses play a pivotal role in Jewish marriages, signing the tena’im (engagement contract, if there is one); the ketubah (marriage contract); witnessing the kiddushin (betrothal) of when the groom puts the ring on the bride’s finger and hears the words he says; and ensures that the couple are alone in the yiḥud room. The same two people can witness all of these stages, or the honour could be given to different people.
Regardless of how many people watch the ceremony, at some Conservative synagogues and at all Orthodox synagogues, the witnesses must be male, traditionally observant Jewish adults (that is, over bar-mitzvah age, keep the Sabbath and keep kosher). The witnesses cannot be relatives of either the bride or the groom, and they cannot be related to each other. In that way, they have no emotional, social or economic stake in the marriage.
The witnesses stand where they can see and hear the bride and groom, and the bride and groom can see them. They may be asked to examine the wedding ring and testify it is worth one prutah (the smallest coin used in ancient times) before the groom can give it to the bride. The witnesses must see the ring given by the groom to the bride, hear him say the marriage formula, and see the bride consent to the marriage.
The witnesses also validate the yiḥud, by determining the room is empty, watching the bride and groom enter, and standing guard to ensure it remains private until the wife and husband emerge.
Some Rabbis permit the English half of the ketubah, and the provincial registry, to be signed by anyone, male or female, relative or not, and also let the bride and groom sign that half of the ketubah. Others consider signatures by anyone other than kosher witnesses as potentially invalidating the ketubah; for them, only a completely separate English document would be acceptable for the bride and groom (and others) to sign.