Open Manage | database | Relationships and note how there is a separate table occurrence box of Events_Guests for every status. Each uses a calculation field in Events that returns one specific status value to match to the status field on in Events_Guests . This makes it possible to define calculation fields that use the count function to count the number of related records with each status.
Thus, for each new status value, you'll need to add a new calcualation field in Events that returns the new status value. A New related Occurrence of Events_Guests and a new calculation field in Events that counts the number of Events_Guests records of that status. You can examine the existing calculation fields and relationships to see how this was set up.
If "table occurrence" is an unfamiliar term, you may want to do this tutorial to get a better understanding how the "boxes" in manage | database | relationships control the function of FileMaker databases: Tutorial: What are Table Occurrences?