"I couldn't sleep last night. I bet you guys couldn't either".
Does the second sentence mean "I bet you too, guys"?
Is it correct to use "either" like that or is it just slang?
When you want to say that you, or someone else, has done the same as someone else, you use too, indeed.
However, in the negative, if you want to say they have not done something, just like someone else has not done it, you use either.
I enjoyed watching the game. I bet you guys enjoyed it too!
The game was boring, I could not watch it till the end! I bet you guys could not either!
Alternatively, you can substitute neither for not either (thanks WS2):
The game was boring, I could not watch it till the end! I bet you guys could neither!
If you would use too in a negative sentence, it means something else:
I scored 1000 points in this game. I bet you cannot do it too!
Notice that now, I did score those points, and I am betting that someone else can not do it.
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