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**!

But:

> The game was boring, I could not watch it till the end! I bet you guys could not **either**!

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.