Your Checkbox need to have a Value list attached where one of the options is Yes. Then all your calculations need to be in your table or a related table.
to see how this works, make a duplicate of your checkbox field on the layout and change it from a "checkbox set" to an "Edit box". Then, in browse mode, experiment by checking boxes and seeing the results in the edit box.
In your calculation, you'll want to use something like ...
... and PatternCount(Response; "Attending Name") > 0; "Yes"