Related על ברכות 7:4

Regardless if [a person that is leading the Zimun] said Nevarech (Let us bless) or if he said Barchu (Bless) we do not stop him [and correct him] for this. [Only] cavilers stop [the leader] for this! [A group] of twenty [people] can split up [into two groups of ten people], as long as not even one person gets excluded from the Zimun.