Shared Spaces can be added to other Organisations. To add a Space to another Organisation you will need to join it first. Go to the Space and click +Join . Now you're able to Add it to any Organisation you are part of.

From the drop down select the Organisation, and select Add. The dropdown will show you all the organisations you are part of and have permission to add a new Space.

By adding a Space to another Organisation you are creating a shared place for the members of those 2 or more organisation to meet. It is a unique opportunity to work on stuff together.