Hello, replying as a recent convert myself. Start with something a bit less daunting of his than WoK but still substantial like Mistborn so that your friends get a taste of Brandon’s writing, which should get them hooked. Then start talking about WoK, the Stormlight Archive and Brandon’s writing in general and just how great it is, so that they feel like their missing out on something then, present it to them again now that they are enlightened to the amazingness (it’s a real word of I say it is) of the Cosmere!
if this doesn’t work maybe try getting new friends? ( also I have literally only been reading Brandon Sanderson for a month so I am not exactly an expert on matters such as this so perhaps it is not best to take my advice?)