Is it a scam?

do not even think about 'signing up with'.

It is a "pyramid scheme" and very illegal. 

https://en.wikipedia.org/wiki/Pyramid_scheme

Yes, it is a scam. Sadly the person who is promoting it to you may not yet have figured out that it is a scam, or that it is illegal.  Basically the person, or business,  who starts it makes a killing. The people on the second or third tier might make money or just break even. After that the person who started it typically stops communicating. 

Those on the next level seldom break even, and everyone after that looses. I am guessing that your friend is on the third or fourth tier. The person who first started it, or even the second tier members have likely already exited the queue, and closed their Facebook accounts.  Those who got suckered in will always say, "it is a promotion" and pressure you. Odd are he or she will keep extending it until he or she breaks even, gives up, or gets arrested.

Odds are he or she was promised a big TV, game console, or maybe even a car if he or she was able to get enough members.

I have seen these many times.  Some accept Amazon gift cards, but most want you to pay using cash, or Western Union.

Facebook is loaded with scams.  Nothing about Facebook legitimizes a business.  There is no reason to trust anyone or any business over another person or business just because they have a presence on Facebook. Anyone can pretend to be someone else on facebook.  There is no validation process built into Facebook.

I am not sure to whom you can report it. The BBB is not law enforcement, and you can only report BBB members.

don't even think about 'joining'

Yes...your friend does not make any money ack until they get 25 other peopkle tosign up and pay $300.

This is called a 'pyramid scheme', and unless it is a VERY GOOD FRIEND...I'd report it to the BBB.

don't even think about 'joining'

Probably a scam! Take no part in it

You already know the answer to this.

Yes, lose your 300 or keep it.