I translated above sentences as follows using a dictionary and the text book which gave those sentences.
A: We are going to eat lunches, Do you also come together (with us)?
B: Good, then we(including B and A and A's friend or smth) go together right?
After searched usages of 吧， I knew at least there are 2 meanings of it.
It makes instruction less direct.
It asks for agreement or confirmation.
So in this case of my post, the first usage is not relevant now.
About the second usage, I've only seen the cases where "?" mark is shown. But in the sentence which I've written at top of this post, there is no question mark.
So how should I interpret this 吧 which said by B?
I guess that the question mark is omitted since the degree of asking or confirming is too weak so "?" can be removed.