I think そちら refers to "you" or "your side (including the second girl's 後輩)".
そちらからも means that the first girl's group will provide 補充員, too.
ほかの後輩にも and their wordings show that the first girl is the second girl's 後輩, and the reason why the second girl says ほかの後輩にも声かけておくね is that 補充員 is still not enough (or the more, the better).