Get author ids from loop

an array of all author ids of the found posts

try:

$all = get_posts('posts_per_page=-1');
$author_ids = array_unique( wp_list_pluck( $all, 'post_author' ) );
var_dump( $author_ids );

http://codex.ww.wp.xz.cn/Function_Reference/wp_list_pluck
http://php.net/manual/en/function.array-unique.php

depending on what you want to achieve, possibly also review
http://codex.ww.wp.xz.cn/Function_Reference/wp_list_authors