If you want your users to be subscribed to a certain subscription automatically, you can use 'Give For Free To Old Users' and/or  'Give For Free To New Users' options.

If you enable 'Give For Free To Old Users' option, then all the users who registered before the subscription plan was created will be subscribed to the subscription.

grant to old users_setting

 

The user is notified about given subscription after he logs in:

grant to old users

 

If you enable 'Give For Free To New Users' option, then all the users who register after the subscription plan was created will be subscribed to the subscription.

  grant to new users_setting

 

The user is notified about given subscription upon registration:

grant to new users

If you enable both 'Give For Free To New Users' and 'Give For Free To Old Users' options then the subscription will be given to each user who registered before the subscription plan was created and each user who registers after the subscription plan's creation time.

Note that in order to have ''Give For Free To New Users'' and 'Give For Free To Old Users' options work, 'Granter' plugin must be installed and enabled.