Hi @gsi thank you for posting in the community. That is a great question! If you are hoping to share the custom shape library with other users on the same team as you you can easily do this by opening the custom shape library by clicking '+Shape Library' and then toggling the 'Share' option found in the top right corner of the shape library. 

Once you share this your team members will be able to locate it in the 'Shared with me' section of the Insert Content window and use it in their documents. 

If you are hoping to share the shape library with users outside of your team this unfortunately isn't currently supported. If you would like to see the option to share custom shape libraries with all users would you mind adding your idea to our feature request form? Ideas submitted to this form are passed on to the product development team for their consideration.

As a workaround you can share a Document with them with all the shapes in one of the document pages.

Once your collaborator accesses the document they can go ahead and add those shapes to their own custom shape libraries by following the steps outlined in this Help Center article. 

I hope this helps! Please feel free to let us know if you have any questions about this.