Employer-to-be offers you to join their social network group and is in fact more than encouraging you to do so. Problem is: you do not work there yet. What would you do and why? I am pretty sure it is hardly a place to discuss business, but who knows, who knows. This is not the test of the CFAI SoE, but the references are welcomed!
I don’t see this as a question of ethics at all. It isn’t unethical to join or not to join the group.
Generally I’d be happy enough to join a LinkedIn group but not for Facebook or anything else more personal. But that’s just me. Depends on the kind of firm you’re joining too. If everyone else in the firm is in the group then you more or less have to do it realistically.
Go ahead and join. Seriously, its a personal preference not Ethics thing.