When sharing your Teamup calendar with others, the best way to maintain security is to be careful to assign appropriate permissions for each user.
If you are organizing users into groups, it's important to check that the group has appropriate permissions set as well.
A user who is part of a group will have the calendar access which is assigned to the group. If the group has Modify permission for all sub-calendars, every user in the group will have that access level.
If a user has one level of permission and is put in a group with a different permission level, the higher level of permission takes precedence.
Here's an example:
User Jones is added to the calendar and given Modify permission for his own calendar and Read-only permission to all other sub-calendars.
He is then added a "Supervisors" group. This group has Modify permission for all calendars.
As long as Jones is in the Supervisors group, he will have Modify permission for all calendars. The higher group permission level will override the lower individual permission level.
If Jones is removed from the Supervisors group, his access will revert to his individual permission settings: Modify for his own calendar, and Read-only for all other sub-calendars.
So, when working with groups, be very careful to check that the group permissions are appropriate for all the users added to that group.