Querying by meta key and value

You can make use of REGEXP operator in META_COMPARE.

For example:

$meta_array[] = array( 
    'key' => '_item_allowed_companies', 
    'value' => $allowed_company, 
    'compare' => 'REGEXP' 
);

$args = array( 
    'post_type'         => 'item',
    'post_status'       => 'publish',
    'posts_per_page'    => $posts_per_page,
    'paged'             => $paged,
    'meta_query'        => $meta_array
);

References:

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