When any user with the Contributor role adds or updates an Activity it is given a "For Review" flag in the background. When an Activity is flagged in this way, is it still stored in the database, and visible to everyone in the team, but will be hidden from Guests.
Any user with the Admin role can remove the flag. To do so, they simply need to open and review the record, and then click the save button. This will remove the flag and the record is now visible to Guests.
If you decide that the Activity has no value appearing on a Guest's Timeline - but you'd still like it stored in the database for internal use - then you can always set the "Hide from Guests?" field to "Yes". This will prevent it from appearing for a Guest user, even after it's been reviewed.
At the time of writing this we are working on some video tutorials to explain this article in detail, with step by step instructions.