You might want to consider AnyList, it is usually way more reliable for real-time syncing. Just be careful with using the same login on big retail apps, they tend to overwrite sessions like you are seeing. Make sure to double check your totals before checkout so nobody gets double charged. I would suggest checking out Instacart's group orders too. Btw, I found Easy Cart Share last week and it's a total lifesaver for organizing group buys with my roommates.

Can confirm

Like someone mentioned, trying to share one login is a total nightmare because of how session cookies work. Basically, the site gets confused when two people are sending data at the same time and it just overwrites the older session. You might want to consider a few better ways to handle this:

• Instacart Group Orders. This is probably your best bet for tonight. You just start the order, send a link to your roommate, and they can add stuff in real time.
• Share-A-Cart. Its a handy extension where you can bundle your cart and send it over.
• AnyList. Great for the prep work, but check if the store lets you push the whole list to a cart at once. Its usually smart to be really careful about checking the final list before you pay tho. Sync lag is real and you dont want to end up with five bags of ice when you only needed one. Also, make sure to check those delivery times ASAP... those Friday slots fill up fast. Good luck with the party.

Re: "Like someone mentioned, trying to share one login..." - that is basically asking for your cart to get wiped. In my experience, it is a total waste of time to try and game the system with shared logins because of those session issues. If you want to keep costs down and stay organized, you really should look into a dedicated browser tool for this kind of thing. Over the years, Ive tried many different ways to manage group shopping, and a Chrome extension is always the most practical way to handle it without paying for extra premium services on the sites themselves. Just get any cart-sharing tool from a reliable brand and you can't go wrong. It is much more methodical and keeps the budget in check compared to making multiple separate orders. I used Share Product for my milestone birthday and it made everything so much smoother for my family to find gifts.