There were very crowded too. 錯甚麼?

錯在沒有主詞。there是副詞。
There were many people in the room.主詞是名詞people
there were應接複數名詞的主詞，但crowded不是名詞，不能當主詞。這是一個倒裝句，主詞在後面。

The room was very crowded.
當主詞補語的crowded是形容詞性質的過去分詞。

文法上是沒錯,只是語意(單句)不清.若是有上下文來襯托,就是好句.

比如說:

一堆朋友相約吃飯後去夜店消費.
A 群人去夜市
B 群人去club hopping

相遇後,B對A說:

There were crowded too.

= those clubs were very crowded as well.

文法上來說:there可以當副詞,代名詞,也可以當名詞("那些地方,狀態,或是地點"的意思).
我上面舉的例子就是當"地方"的意思.

另外更好的說法是:

It was super crowded there.

crowded 什麼？