The quick answer is "no".
Coins sent to an address belong to the "address owning" wallet after the transaction. And only the person who can unlock the wallet can do something with the coins. Thus, if it isn't an address of your wallet, you have no access to the coins transferred to that address. And if you didn't tell someone else your seed and wallet unlocking password, only you have access to coins in your wallet.
Or do you mean something else with "can anybody have access"?
Thanks very much for the response.
It's good to hear that someone else thinks at least a little similarly to me regarding the UI and clear as mud info displayed.
It's disheartening to begin learning that the developers haven't put a huge amount of thought into the overall user experience (I was hoping my issues were from me just being dumb initially). Don't get me wrong, I think Sia is great and a lot of good work has been done in a short time frame, but ease of use is very important when attracting and retaining new customers.
It would be nice to get a response from a known dev to confirm that the problems are understood and likely to be addressed. Is there a more prominent place I should post this?
I've invested a reasonable amount into this technology now because it's genuinely such a brilliant idea. It would be fantastic to see Sia become a world standard for safe secure cloud storage. Their success will be measured by how much the devs listen to end user gripes and ideas for improving the user experience.