Previously I wrote about one way to do this. Having experimented, I've found another. You start by sharing your site.
The person gets an email with the link in it
If they do not have an account, and the presumption is they don't, then they need to create one, so they click the 'sign in with a different account link'.
On the next screen they click 'create an account now'
This takes them to the new account screen. Fill in the details - use your own email address, the one the invite was sent to
do the captcha and it takes them to the Site
When returning to the Site, the visitor does not need to create an account, they login by clicking 'sign in with a different account'. Even if you share multiple Sites with them, a single login only is required.
Also, if you have shared multiple Sites with the same person, they can navigate around by clicking the 'Sites' link at the top right
which takes them to a summary page