WP_User::add_cap( string $cap, bool $grant = true )
Add capability and grant or deny access to capability.
Parameters
- $cap
 -  
(string) (Required) Capability name.
 - $grant
 -  
(bool) (Optional) Whether to grant capability to user.
Default value: true
 
More Information
Changing the capabilities of a user is persistent, meaning the added capability will stay in effect until explicitly revoked.
This setting is saved to the database (in table wp_options, field wp_user_roles), so it might be better to run this on theme/plugin activation.
Source
File: wp-includes/class-wp-user.php
public function add_cap( $cap, $grant = true ) {
		$this->caps[ $cap ] = $grant;
		update_user_meta( $this->ID, $this->cap_key, $this->caps );
		$this->get_role_caps();
		$this->update_user_level_from_caps();
	}  Changelog
| Version | Description | 
|---|---|
| 2.0.0 | Introduced. | 
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Licensed under the GNU GPLv2+ License.
    https://developer.wordpress.org/reference/classes/wp_user/add_cap