You might have a new person on your team that needs access to your project. This a manual process to ensure only the right people can access the project, containing sensitive data like customer's addresses and payment methods.
If you want to add someone to your project in Firmhouse you simply have to visit the 'Team Members' settings and invite a new member using their email address.
An email will be sent to the person and they can accept the invite to be added to the project.
Once the team member has accepted the invite their role can be edited to either remain a team member or a project manager. Project manager roles provide more administrative capabilities like changing the price of the subscription.