How should I interpret this 吧 in the sentence "好，那咱们一起去吧" ？

Question

A：我们去吃午饭，你也一起去吗？

B：好，那咱们一起去吧。

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.

1. It makes instruction less direct.

2. 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.

Answer 1

吧：OK

A：我们去吃午饭，你也一起去吗？
A: We're gonna get some lunch, wanna come with？

B：好，那咱们一起去吧。
B: Yeah OK, I'm in.

Answer 2

I think it should be like this：

A: 我们去吃午饭，你也一起去吗？ A: We're going to lunch, wanna come with？ B: 好，那咱们一起去吧。 B:OK, let's go together then.

In there, '吧' is express affirmation

Because A is asking B whether to have dinner together, and all B has to do is answer.

Answer 3

A：我们去吃午饭，你也一起去吗？
B：好，那咱们一起去吧。

这里的"吗","吧"，是助词，语气助词，让语气缓一点，避免说的太快，避免给人急促的感觉。
I am sure you can say this way:
A：我们去吃午饭，你也一起去？
B：好，那咱们一起去。