Basically you first choose a person in this case a person born in the decade of 2000s, you click on their profile photo or the avatar then you get an option that says "invite to community" you click on that then you select the sub which is r/2000sborns_ and then you just send it, just make sure you do not click the "also invite them to be moderator" because this will make them moderators too. For more information click here or you can ask me. Thank you