They find out when they get their start up packet from the company. People will do this for monthly payment programs and the dealer receiving the packet get's their first bill for something they didn't join. If they don't promote the program, they hear from their upline complaining. Sometimes dealers are bought into things from total strangers. Other times its from friends, which makes it worse.
Many dealers feel obligated to promote it when it comes from a friend.
Let me put this as nicely as possible. It is wrong to try to use mail order friendships to try to make you money. Do not buy anyone into opportunities without their permission. Each one of us has a unique business. We all have certain goals for our business.