We
Us
pronoun
When たち is attached to a word referring to a person, it makes it plural. So, when you attach たち to 俺 (I), it means we or us.
Just remember that 俺 is quite casual and rough sounding, so it's best to only use it with close friends (or when you're shaking someone down...).
The reading is the same as the one you learned with the kanji. You should be able to read this on your own.
俺たち、一年生です。
We're freshmen.
それ、俺たちのバイクです。
Those are our motorcycles.
誰もやらないんだったら、俺たちがやるしかないだろ！
If no one else is going to do it, we'll have to!