What I'm about to describe is how Amazon.com works, when I ship to my US address for parcel-pickup just across the border. For all intents and purposes, according to Amazon.COM that is my primary address. Now two friends of mine also coincidentally use the same parcel service at the same address.
I'll tackle your specific question a bit further on... but how *WE* do it, is we split the cost of Prime (we pay cash to the primary holder, out of pocket). One of us is the primary Prime member... and through Amazon's Prime account management option, the primary member invites the other two as members of the same address. After we do a manual "handshake" (for lack of a better term) through Amazon, Amazon then identifies that a.) we know each other, and b.) we have the same primary address. Each of us maintains and uses our own Amazon accounts and we can make our own discrete purchases with our own accounts using our own payment methods for our purchases, and the parcels are all shipped via Prime to the single address in the US.
I haven't confirmed this, but I believe that Amazon.ca would work just the same way... but that's having your separate accounts intact, ALL shipping to the same address -- which I completely recognize is not quite what you asked... but I told that for a reason so I could enhance that explanation...
To address your first part of your question... (still using .com as the comparison)
Technically you're supposed to have only one address that you have items shipped to -- however (still referring to my US-based Amazon.COM account) while vacationing in the US, I have had parcels shipped to my hotel, in a completely other city in a completely other state than my "primary" US shipping address... and they shipped via Prime to my hotels... no questions asked.. no issue and no complaints from Amazon. However I don't do it all that often. (and it has been done regardless whether I was the primary Prime holder, or if it was one of my friends at that time) -- as well I have also successfully shipped to an Amazon "pickup location" in the states and had it shipped Prime with no issues.
So in summary the criteria that works above (again using .com) is that
- Each of my friends and I ALL use the SAME primary shipping address on our accounts. (our billing addresses are all in Canada, and are all different, incidentally) -- but fortunately 99 times out of 100 we're all normally shipping to that single address anyway.
- My hotel, whenever I add one, gets listed under my account as secondary address Regardless, the parcels still ship via Prime methods
- My friends alternate US addresses, similarly are listed as secondary addresses without issue.
- I have also successfully shipped to an Amazon "pickup location" in the states and had it shipped Prime
Shipping to an address OTHER than your primary address seems to work using .com and you're able to share your Prime account with other people who are 'allegedly' in your household and have the same primary address.
[*Another benefit of doing it THIS way is that if you want a lightning deal (usually limited to only one per person), each of you STILL gets a crack at it. Whereas if you use ONE account for all the purchasing you are only eligible for ONE lightning deal. -- which can be a problem especially around black friday deals.]
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Now, focusing on Canada... Amazon.ca
While briefly having Prime in at Amazon.ca I have successfully shipped parcels to family members homes and to my satellite work office in another city -- and in all cases they shipped via Prime methods. -- But again, I didnt do it often, just a few times in total. But was never blocked and never heard from Amazon about it.
So you COULD do it that way if you wanted. But I would suspect if you made an obvious habit out of shipping to alternate addresses, it might raise some red flags.
[Note: I usually only ride the 30-day trials whenever Amazon.ca lets me again (usually once a year). I have never paid for Prime in Canada nor held it long enough to make specific comparisons with the US model.]
Presuming Amazon.ca follows the same policy, practices, and procedures as the US where it pertains to sharing Prime, you could try the following:
If all your friends are local, see if you can SHARE your Prime account (again, I haven't confirmed if you're able to do this in Canada or not). Before you start the invite process, be sure and all have your primary shipping address (billing doesn't matter) set to the same address. This way, each of them has their own discrete cart, payments, etc... They can place their orders as they see fit, and have them shipped to that common address.
As I say, I wouldnt make a habit of shipping them off-the primary too often.
I know it doesnt give you a clear answer to your question. All I know is how the US model works, and my limited experience with the Canadian one. But still, I hope it helps a little.
ADDENDUM: Not that it matters here in Canada, but when using .COM, the secondary "shared" members do NOT reap any other benefits of Prime OTHER than shipping. Only the person whose account is billed for Prime can access the Prime Video and cloud services. -- which with Amazon is more complicated gaining access from Canada that it is accessing US Netflix. With Amazon it's more hassle than it's worth because you need a credit card with a US billing address, and more than just a Proxy or a VPN to make it happen anyway. BUt thought it was worth mentioning, should Canada get into the additional services too...