Fellow detects a 1-on-1 in your calendar any time that you have an event with only 1 person. You can change your Notification Settings to prevent getting notifications about meetings that are not 1-on-1s. To do this navigate to User Settings --> Notifications.

You can set your ‘Upcoming 1-on-1’ notifications to only notify you about a particular recurring meetings in your calendar, such as your 1-on-1s.