Bitrix24 calendar owner can set various access permissions to different users inside calendar settings. To open settings please navigate to Calendars:
In My Calendars list select calendar you'd like to configure, open calendar options & click on Edit:
Please note that if you set access permissions All Visitors = Full access, the further individual access limitations (e.g. user X= access denied) will not work. Access Permissions in Bitrix24 are based on the following rule: the user is always granted the highest level of access allowed by the various permissions.