assembly required tables

Question

1. Can one open the legs of a bridge table on shabbat (to eat on, or some other use)? 2. Can one add planks (proper word?) to a table on shabbat to accomodate more guests at your shabbat table? 3. Can one put together a table that requires some assembly (for example, putting together the top of the table with the legs using simple screws)? 4. If any of the above are forbidden, why? 5. If forbidden, and you are a guest on shabbat, and you witness your host (ostensibly shomer shabbat) violating this law -- are you: a) obligated to inform them of the violation? b) obligated to leave? c) any other issue the Rabbi deems relevant to address here... Thank you very much!