How to add extra users & customs users?

Hello!
1. How do I add extra users (over 5) - need to add more sales reps who can do invoices & dispatches, see details for their sale contracts but not everything there is on the firm.

2. How do I add custom users (where I can customise access levels)? Help file (accessed from inside QB) mentions that I need some sort of subscribtion over "Plus" level (what we have now), but I see no way to upgrade.