This question has been asked in more specific ways in the past, and I have read them all, but I'm asking as a general concept (if that's possible). Here are the two options:
- subdomain -- https://acmecompany.webapplication.com OR
- user id -- https://webapplication.com/324329/projects
The upside of #1 is security, the downside is the creation of the subdomain and SSL for each.
Note: I'm still thinking through the question of 1 database to rule them all, or individuals. But that's a question already addressed brilliantly in other threads.