Default Groups - How to add existing users (2k) without sending email notification

Hello,

We have two Profile Types - 1) Members - 1746 users and 2) Mentors - 18 users.

We have just added 2 default groups for users to automatically join when they register.

1) Will users have a choice whether to join the group?

2) Is there a way to have the 1,746 + 18 users join these groups WITHOUT kicking off an email notification 1,746 + 18 times?

Thank you!