Here is a replacement Groups Cache File, that is totally empty. All you need to do is download it, extract it onto your desktop and copy it into your Sims 2 Folder. Now the trick is to right click it and then click properties, the box at the bottom needs be ticked next to Read-only, then click apply
If the game cannot write to it, it cannot corrupt it. Why keep a copy on the desktop? For if and when the game somehow writes to it again and refuses to start, you can just replace it.
http://www.mediafire.com/file/admhh81bsyyhda4/Groups.7z
or do as I do and just open a new browser window and copy and paste the following:
mediafire.com/file/admhh81bsyyhda4/Groups.7z
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You have two chances, slim and none, but slim has already left town.