Neither being a member or being accompanied by a member is required to walk in the door. You don't even need to be a member to shop there provided you have a Costco Cash Card [0]. As well, there are at least a few Costcos where the concession stand is outside [1]
So if I give my Costco-member friend $100 and ask her to buy me four $25 Costco Cash cards, would that let me make four trips without buying my own membership? That seems like all I'd need in a year...
This is accurate. You need only to have a costco cash card to get the Costco price on the goods in store. That cash card does not need to encompass the entire price of the transaction, as your credit card can make up the rest.
I've heard this touted as a "hack" but this makes absolute economic sense for Costco -- they get an advance on your purchase and get you into the store to potentially see the value in signing up.
I tried that for a while in lieu of membership. Their employees certainly gave me a hard time over this: stink eye, calling the manager, chiding me for doing this loudly and openly, etc.
Thus, while it is possible, their devout employees don't like it and go out of their way to make one feel like a leper for not being a member.