When you create a prospecting or ticket management campaign, it is a team of users who will take care of the action processing.
Therefore, it is therefore teams and not unique users that you will assign to the processing of a prospecting campaign or tickets (How to assign a team to a campaign).
This way, a team will be able to see certain campaigns that other teams won't see.
To add a team in your domain, go to the Domain management menu > Manage teams and click on the button