It's important to know the structure of the table that forms the context for your calculation. I think that you have a record with these fields:
TableNumber, GuestID (or name), numberOfGuests
So if George Smith, party of 2 and John Johns, Party of 3 are assigned to Table 1, you have two records, one with 2 in the numberOfGuests field and one with 3.
If that matches your table design, then you can add a summary field that computes the total of NumberofGuests.
A summary report layout can then be set up for list view where you Add a sub summary layout part "when sorted by TableNumber". You would then place the summary field inside this sub summary layout part and, as long as you keep your records sorted by TableNumber, it will show the total number of guests at that table.
For more on summary reports, see this tutorial thread: Creating Filemaker Pro summary reports--Tutorial