您只需要使用别名（例如user2）对表用户进行另一个内部联接

SELECT users.firstname,
    users.lastname,
    users.username,
    users.image_url,
    posts.status,
    posts.created_at as created_at,
    posts.updated_at as updated_at,
    share.post_user_id as user_id,
    posts.id,
    share.message as message,
    share.user_id as share_user_id,
    users2.firstname,
    users2.lastname,
    users2.username,
    FROM `share` 
    INNER JOIN `posts` ON share.post_id = posts.id 
    INNER JOIN `users` ON users.id = share.user_id
    INNER JOIN `users`  AS user2 ON users2.id = post_user_id.user_id