As an Admin, you can invite users to individual projects from the "Invite team members" button in the header. New members are added in the "User" role.
You can control the role of your fresh invitees and existing Hutte organization members from "People" in the organization settings menu, which is accessible from the cog symbol located in the top right corner.
Assign Project Admins
Hutte organization admins can assign individual users as "Project Admins" for specific projects. This grants them comprehensive project administration access. However, they cannot create new projects or access other projects unless they are assigned roles in those. You can make these changes via the "Project Settings" screen on each individual project.