You could try looking in your particles folder and looking for any duplicates.
This is what causes most crashes.
Did you download any mods lately? Perhaps there's a misplaced file that's causing issues
And it could simply be that your computer can't handle the lag caused by all the users. Try changing your rendering system, lowering your view distance, turning off water reflection, or all three
Did you download any mods lately?
Is your internet fast and strong? Check to make sure it isn't going through a temporary derp as well
It could also be that you have anti-aliasing turned on. Make sure that in your video settings you have "Anti-Aliasing" turned to zero. You could also try lowering the resolution of the windows in your settings before trying to enter The Grounds.
'Ave ye checked f'er missing map files? It could be that The Grounds map file is missing.