This is a thing that happens: you've been playing D&D for a while with some cool folks. Their characters are climbing the ladder of levels, gaining hit points and cool abilities and all that stuff that signals accomplishment in D&D.
And then, hearing what a good time this game is, a friend asks if they can play too. They've never played D&D before--hey, maybe they will have so much fun you'll make a convert of them!
But...it might be pretty overwhelming to have them make (or worse yet, just hand them) an experienced character that fits the party's level. All those numbers and abilities that feel like accomplishment for the regular players in your game might feel confusing and hard to remember for a person new to the game. Hell, just the stuff a 1st level character gets can be a little daunting.
A solution that I have not yet stress-tested: help them make a 1st level character, but give that character the hit points and proficiency bonus equal to where the rest of the party is at level-wise. If the rest of the group is playing 5th level characters, help them make a 1st level character who has the hit points and proficiency bonus of a 5th level character. This way, they'll only have to grapple with the abilities and options of a first level character, but have some of the survivability and competence of a higher-level character. As they level up, they won't get more hit points or a higher proficiency bonus until they level past where the group was when they joined the game, but they can gain new abilities as they level up--hopefully slow enough that it won't feel like an avalanche of new stuff to keep track of.
Best case scenario: they level up more rapidly due to facing challenges appropriate for higher level characters, they still have fun because the higher hit points mean they don't die in one hit and the higher proficiency bonus means they still get to meaningfully contribute to the group's endeavors, and they have such a good time that they begin to enjoy their character and want to keep playing.