Does anyone know where we can find an ERD for the ORION DB (Primarily SAM) Application/Group Members/Nodes, etc?

You won't find it.  It's something that changes version to version.  This is why we suggest people use the SolarWinds Platform API if they need to query the data sets.

Never too late..... Unfortunately no ER diagram or data dictionary, however SolarWinds do provide SWQL by mean to  manipulate data, see below there is a very useful namespace and take a note of summary column. 

head hear for more information https://github.com/solarwinds/OrionSDK/wiki

@ backrooms Check the official documentation provided by the developers of the ORION DB or SAM application. This is often the best source for accurate and detailed information. Engage with community forums or developer groups related to ORION DB or SAM. Members might have shared ERDs or similar diagrams in the past.