Wordpress - How to get WordPress Username in Array format

The other answers are correct, but it's possible to achive the same thing with less code using wp_list_pluck():

$users = get_users();
$user_names = wp_list_pluck( $users, 'display_name' );

wp_list_pluck() used that way will get the display_name field of all the users in an array without needing to do a loop.

The get_users function will give you an array of user objects, from which you can extract an array of user names. Like this:

$args = array(); // define in case you want not all users but a selection
$users = get_users( $args );
$user_names = array();
foreach ( $users as $user ) {
    $user_names[] = $user->user_login;
}

Now $user_names is an array with login names. You can, off course, also use user_nicename, last_name, or whatever info is available in the wp_user object

Look at get_users() function.

<?php

$users = get_users();

foreach( $users as $user ) {
    // get user names from the object and add them to the array
    $get_arr_user[] = $user->display_name;
}

And you'll get the array similar to following:

Array
(
    [0] => John Doe
    [1] => Jane Doe
    [2] => Baby Doe
)

I'm pretty sure you'll want to exclude admins, order names and so on. So, look at the documentation to find out more get_users() arguments.