Adding 咱们 to the start would make it even more obvious.
咱们一起走吧！ is also correct. :)
Isn't 我们 more common?
Depends on which part of China you are from. It is probably less common than 我们 in Southern China than Northern China.
is 出去吧 ok？
I think that means let's go out, which is not quite what was asked here
I got that marked wrong on Jan 27th, 2020.
"Let's go" doesn't necessarily mean "going out". That's not a good translation.
走吧 can also mean get out of here!
I'd say “滚吧” in this case. 走吧 sounds way too polite.