We wouldn't want to make the project public due to security reasons, but we also wouldn't want to buy those people additional licenses as they will only look at the tasks and discuss with us, therefore they will not perform. any actions.

Workaround

As a workaround, you could allow anonymous access to projects (not possible on Free plan) but asses with your team and consider the security implications of such change before applying it.
Optionally, there are a few third-party addons in our Marketplace that could help you share Jira data externally.