If you want a new user, or set of users to belong to a group, you assign that with the -G option. This is assuming they will have accounts on the box and access to the box. If they are virtual users (samba users, website logins, virtual email) then they don't need to have actual accounts and don't necessarily need to belong to the users group (-g users).
To give your users permission to a specific folder, and only certain users, that's the case where a special group (projuser) would come into play. Not every user on your system necessarily needs access to that folder, so you give specific users access by adding those users to a group you've created (projuser) and own that folder to that group.