HI Gerardo,
Sorry, sometimes I make wild assumptions… 
Below is a link from the main table, to a table on the canvas. In practical terms, the formula reads the Attendees table which is on the canvas of the canvas column, and count the number of attendees for each meeting. (Take care to select the table, and not the row specific view. If you choose the latter, the formula will only work for that particular row of the meetings table, and not all the other rows as well. Just replace count() with sum() and you will be ok.
The below is a link from the table in the canvas, to the table of which the canvas is a column. (I.e. Meeting nr 6 is the value of the display column in the Meetings Table. The filter formula selects the people from the Attendees table who have been invited to Meeting Nr 6.
Please let me know if there is anything that os still unclear.
Regards
Piet