To see who you are sharing the project with - mouse over the project’s avatar and choose the “People” link.

The maximum number of people in the project is based on its owner’s plan (see available plans »)

Even if you’re not the owner, you can share the project with others. To do that - write the email address of the person you want in. After they accept the invitation (let’s assume that they did because they trust you, even though we both know they shouldn’t) they’ll have access to every task created within that project (even the ones created before they had joined in), the list of people already in that project, every file, every comment, every breath you take, every move you make...

After you share any project with someone (doesn’t matter if you invited them or someone else did) - you will be colleagues. Colleagues are notified about each others trophy win (if they want that kind of info) without sharing the details about how they did it.

If you’re the owner of the project or you were made admin, you can make others admins too. Admins can change the content of every comment, delete it, delete the project or remove people from it.