From memory, the saving when you buy the full alowance of entry and camping tickets roughly equates to the cost of membership.
With membership you also get 10% discount on all goods at the ACO shop, access to the ACO buildings (noted for it's showers and bar), the ability to request seats in the ACO grandstand (but these arent cheap at all) and a very limited breakdown recovery service throughout France and other parts of Europe. Ticket discounts apply to all major events at the track, but not to all tickets (e.g. parking/camping at pre-qual was the same for member and non-member, but entry at the 1000km last November was cheaper for members). And the last thing I can think of is a pack is sent to you in the summer with full results, a copy of the official poster and some other bumph.
You get no priority in allocation of any tickets, so the offsetting discounts might not be work out.
I'd be surprised if you couldn't join up down there at one of the ACO outlets and yes, if you did it now you'd be too late to receive the tickets etc before you go. My renewal is in April. If you can get it before the race, I would imagine it would be at the Rotunda - otherwise you would need to have already bought your entry tickets in order to reach one of their shops.
