Posted September 25, 2015 at 6:19 pm

I imagine Sarah and Grace pretty much always invite Susan to participate in whatever they're up to. I also imagine Susan generally accepts if she's available , but there are clearly exceptions.

Granted, that begs the question of why Susan wasn't already there. Hm... OH! Sarah was doing mad science stuff with Tedd earlier, and this is after that. There. Perfect explanation as to why they're only just now inviting her to hang out. TAKE THAT, LOGIC.