Invitation by link
A link goes to the person and the role is assigned at invitation time. No manual account creation.
Access is scoped by project, role and action: tester, reviewer, test lead, viewer. Organisations are isolated from one another at the data level.
Access is scoped by project, role and action: a person sees what they work with and cannot break what they are not responsible for.

Organisations do not overlap at the data level. That matters to a contractor with several clients and to a company where departments should not see each other's projects.

A link goes to the person and the role is assigned at invitation time. No manual account creation.
Authorisation is built on Keycloak, so a corporate account plugs straight in.
Changes on cases and runs reach the interface immediately, with no page reload.
Invite colleagues by link, hand out roles and see how it maps onto the way your team already works.