What are the limitation to people on the project that are NOT a register users?

What are the limitation to people on the project that are NOT a register users?

We have been using 5pm as our project manager for years. Since we added CRM Plus we are concidering moving our project management to ZOHO projects. Is there a limit to the involvement of non registered users of ZOHO. We a majority of our projects and tasks that require outside collaboration with customers and vendors. What can they do and not do in Projects?