Note: The following article concerns sharing private diagrams. If you're on a Team plan and want to share diagrams within your organization, please see the article Projects.
You can invite other users to access your stencils and templates, or put them in a shared folder. The procedure is the same as sharing diagrams, which you can find here.
Then, the people you’re working with can find the stencils under “User Defined Stencils” on the Stencil panel, and the templates under “User Defined Templates” on the Templates panel.