Hi Jonathon

Thanks for posting in the community! Unfortunately there is not currently a subset symbol currently included in Lucidchart's ER Diagram shape library. You can however create your own custom shapes in Lucidchart. For instructions on creating custom shapes please reference the Custom Shapes article in the Lucidchart Help Center. We'd also love it if you'd add this idea to the product feedback section! This is shared with our development team for their consideration in adding new features shapes and functions to Lucidchart. Let me know if you have any questions!