Each experimenter with access to the JATOS server (though the GUI) is a JATOS User. Users can create, modify and delete the studies they are members of. They can also export and delete results.
A study in JATOS is allowed to have more than one users, also called members. Each member has the same rights, e.g. can run the study, create new Workers, add/change/delete components, export/delete results. Especially each member can add new members or remove existing members.
Each study has a Change Users button in its study toolbar.
In this menu you can add single users by their email address. Of course this works only if this is already a JATOS user. For privacy reasons JATOS never shows the email address in the member list.
A single user is removed by unchecking the checkbox in front of its name.
Additionally it is possible to add all JATOS users at once or remove all members at once.