Project admin does not have access in the vRA portal to manage their project: add users and groups, or assign roles in projects.
VMware Aria Automation 8.x Service Broker
Service Broker "user" role does not grant access to the Infrastructure tab in the GUI
This is shown in the service roles docs article, as "See and open the Infrastructure tab"
It is possible to administer projects by API.
An update has been made to the docs article to clarify: a SB user can manage projects "Only via API". In a future version this may be provided through GUI.
Please create a workflow catalog item which submits the project member changes as an API call to Automation.
You can create a form for this which provides project administration options.
The alternate workaround is to grant SB Viewer / Administrator or Assembler User role to the project admin(s)