Costco question

[DrWhy]

I’m am catering for a picnic for about a dozen people on Saturday. I keep being told that Costco is the best place to get things like sandwich platters. I’ve just discovered that I am eligible for membership through my work.
For anyone who shops at Costco, do you know if I can just turn up, sign up for membership and buy sandwich platters off the shelf (out of fridge) or do I have to order them in advance like I would Sainsbury’s? I have a feeling that I might have left this whole arrangement a bit too late!

Thanks for the info everyone. It seems I can just turn up on Saturday morning with my company ID, a bank statement or utility bill, sign up and start shopping and they do have sandwich platters in stock.
Still dithering over whether to throw myself into the delights of Costco or whether given the timings I’m on it would just be sensible to get a Tesco delivery to home and give myself an extra hour and a half to finish making a tractor cake....
The Tesco sandwiches are quite expensive and not especially nice though....!

You never know how long it will take at checkout...