Again with the "we" vs. "let's" translation. The rollover says we, but the answer is Let's? Shouldn't it be both?
When you have 'ni' and then a verb with a -u ending, it can be translated as Let's verb.
Ni iru = let's go
Ni batalu = let's fight
Ni dancu = let's dance
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.
Of course, "let's" is a contraction of "let us," which would translate as "lasu ni." So ni is present.
You can use "we" here.
"We should participate" is a correct translation.
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".