It's part of the verb "einsammeln".
We take the guys in the bus. The rubbish in the bin. We collect things, which are spreaded on a large field in a container like object or a small piece of land. --> einsammeln
While we collect butterflies, stamps, .... we get them by friends, we buy them - we don't need to catch them. -->sammeln.
There are guys, who collect lingots, they like to store them. --> sammeln
And there are guys, who collect lingots, because the lingots fall onto their table, appear on the screen and the persons only need to wish them together or to catch them with their hands (into a bag). -->einsammeln.
I like that, but is still took some time looking at google to understand. perhaps a better way of seeing it is this.
Sammeln; to collect things, as in a hobby, as in, a collector collects coins.
Einsammeln; to gather or collect up things, as in, he collects the garbage for disposal
The subject and verb are inverted.
"Sammeln wir ihn ein" is an imperative. Note that the subject and verb are inverted. You could translate this sentence as "Let's pick him up", but not as "We are picking him up".
Give this a thorough read: http://www.dartmouth.edu/~german/Grammatik/Imperative/Imperativ.html
'ihn' is a singular word, how can one collect one thing? You can collect many things of one object, but even then a German would not use 'ihn'. -- Lassen Sie uns diese Art/ diese Sorte/diese Karten sammeln!"
The 'zu' is wrong in your sentence. I don't know, why you want to use it. The infinitive of German verbs does not have a 'zu'.
Wir sind bereit, es zu sammeln. Here it is a subordinated clause, a construction with 'zu'. ~We are ready to collect it.