If two people are married, or at least are linked as a couple in a Family record, they effectively share all of the Family events, such as Marriage, Divorce, etc. That is one way that two people can share an event. Sometimes, however, you want to record an event which is shared between numerous participants, and it may be that none of them are a couple in any sense. Suppose, for example, that four people all lived together for four years. It would be nice to create a single Residence fact, which they all share. To do this, you need to create a Residence fact for one of them (it doesn't matter which one), and then add all four of them as witnesses (that is, non-principal participants) to that fact, with role of 'Resident' (see How to Record Witness Details). Notice that even the person you originally added the fact to, can be a witness to their own fact. And if they are a witness to their own fact, this will override their status as the fact owner (for purposes like sentence generation in narrative reports, say). What this means is that they will be treated exactly like all the others, in all output. Of course you can treat them differently if you want to, and even give them a different role (if say, they were the tenant). The point is that you don't have to. In practical terms, this means that you can have events which are shared equally between all participants.
In Family Historian, any event which has one or more witnesses, is
called a shared event. See Witnesses
and the Witnesses
Window to learn more.