Set the "bp_docs_create" dynamically when in a group.

docs-create-cap-mirror-group-setting.php

<?php 
function my_docs_create_follow_group_associate_setting( $caps, $cap, $user_id, $args ) {
	if ( 'bp_docs_create' == $cap && $group_id = bp_get_current_group_id() ) {
		// In a group, only set this cap if the user can associate a doc with the group.
		if ( current_user_can( 'bp_docs_associate_with_group', $group_id ) ) {
			$caps[] = 'exist';
		} else {
			$caps[] = 'do_not_allow';
		}
	}
	return $caps;
}
add_filter( 'bp_docs_map_meta_caps', 'my_docs_create_follow_group_associate_setting', 10, 4 );