I'm pretty sure people only offer Discord to the random players after everyone zones in because there's no point kicking your own premade members out of your party and inviting the randoms in and giving them a link to the Discord when it's all RNG who gets in in the first place and you can't really tell who outside of your party is going in until everyone is already in.
Discord is offered after everyone is in because now you're all stuck with each other and you might as well make the best of the situation.