One could also say "Let's join in" or "We're in on it." or even "We will do x"
...or "Let's take part".
Again with the "we" vs. "let's" translation. The rollover says we, but the answer is Let's? Shouldn't it be both?
Ni means we, but in English you don't have the equivalent of the u-form, so you have to use Let's, but you really translate the u by let's, ni still means we and ni never means let's. It is only because of the u that you use let's.
You can use "we" here.
"We should participate" is a correct translation.
This is exactly how I think about "ni -u" phrases.
Of course, "let's" is a contraction of "let us," which would translate as "lasu ni." So ni is present.