Shared calendar across multiple 365 tenants

[TheFungle]

My team have recently started a collaborative project with members joining us us from two different companies.

All of us are currently using O365 but one of the tenants has zero ability to connect with the outside world, with the exception of Teams calls.

Is there any easy way to setup a shared calendar or equivalent functionality that I'm missing?

Moving to a different provider such as Google unfortunately isn't an option.