Unfortunately, Portfolio isn't a required field to create a project so you can't really restrict it anyway and users will have access to create projects from the main project screen.
There are a few workaround ideas that could get your there.
- If you restrict your users to only create projects from a template and those templates have custom forms tied to them, you could create a mandatory field that would prompt them to properly align their project to a portfolio (in theory making the portfolio required. Add Workfront native fields to custom forms
- You could take away the main project screen from their layout template and make their default page the Portfolio page. This way they aren't seeing the new project option from the project screen and it will force them to click in to the porfolio and only see the new project option in the portfolio they have shared with them. However, if your users are using the new home area and the My Project Widget, they will see the option for New Project from that area and this option won't really work.