Only allow certain users to create docs.

restrict_doc_creation.php

<?php 
add_filter( 'bp_docs_map_meta_caps', function( $caps, $cap, $user_id, $args ) {
	if ( 'bp_docs_create' === $cap ) {
    // Site admins
		if ( user_can( $user_id, 'bp_moderate' ) ) {
			$caps = array( 'exist' );
		// If this is a group and the user is a group mod or admin.
		} else if ( bp_is_group() && ( bp_group_is_mod() || bp_group_is_admin() ) ) {
			$caps = array( 'exist' );
    } else {
			$caps = array( 'do_not_allow' );
		}
	}
	return $caps;
}, 20, 4 );