When adding a participant to a communication, its main position is automatically selected. From that entry, you can edit the position and choose another one or none.
Selecting a position will add communication to the organization’s list of communications.
In case no position has to be selected, the option “As individual” must be selected. In that case, the communication will only appear in the individual's list of communications.
This video briefly shows how to proceed: https://www.youtube.com/watch?v=azz3GHKc058