Conversion failed when converting from a character string to uniqueidentifier
This may be a typo in your question, but just to be safe I'll post this as a possible answer - you shouldn't have those single quotes around @siteId, it should be like this:

siteGUIDInAnotherEntity=@siteId


The text string '@siteId' can't be converted into a uniqueidentifier for that join, and will result in the error you mentioned:

SELECT CAST('@siteId' AS uniqueidentifier);



  Msg 8169, Level 16, State 2, Line 1
  Conversion failed when converting from a character string to uniqueidentifier.