Projects, by design, are private in Inspire Planner. This means that the project owner is the only one who has read/write access to the Project. To share a project with another user, you would need to add them as a Project Collaborator.
Another option is to create a global sharing rule that will grant a set of users read/write access to Projects. This involves creating a public group and then configuring a sharing rule for the group. This makes it easier to share projects between users without needing to add them as project collaborators. This help article will walk you through how to:
The first step would be to create a public group that contains the users you would like to grant read/write access to for Inspire Planner Projects.
Once you've created a Public Group, you can now configure a sharing rule for the group.