Get the comment author ID by the comment ID

you should use it: <?php get_comment( $id, $output ); ?>

Return

comment_ID 
(integer) The comment ID
comment_post_ID 
(integer) The post ID of the associated post
comment_author 
(string) The comment author's name
comment_author_email 
(string) The comment author's email
comment_author_url 
(string) The comment author's webpage
comment_author_IP 
(string) The comment author's IP
comment_date 
(string) The datetime of the comment (YYYY-MM-DD HH:MM:SS)
comment_date_gmt 
(string) The GMT datetime of the comment (YYYY-MM-DD HH:MM:SS)
comment_content 
(string) The comment's contents
comment_karma 
(integer) The comment's karma
comment_approved 
(string) The comment approbation (0, 1 or 'spam')
comment_agent 
(string) The comment's agent (browser, Operating System, etc.)
comment_type 
(string) The comment's type if meaningfull (pingback|trackback), and empty for normal comments
comment_parent 
(string) The parent comment's ID
user_id 
(integer) The comment author's ID if he is registered (0 otherwise)

Final Code

$comment_id = $commentID; //$commentID your var

$comment = get_comment( $comment_id );

$comment_author_id = $comment -> user_id;

Use get_comment to return information about the comment like comment_author_email.

You can then try to get a user by email using get_user_by('email', $comment_author_email).

Once you have the WP_User you should be able to access the ID of that user.

All of this assumes, the comment author's email is used as the user's registration email.