Make the people who have an account on your site (members, customers, authors) into contacts in one step. Each contact stays linked to the user account.
Before you start, create the lists you want, and decide honestly whether these users agreed to marketing email. Having an account is not the same as agreeing to a newsletter. See Consent and the law.
- Go to Audience > Import / Export and stay on the Import tab.
- Below the CSV wizard, find the card Import your WordPress users and press Set up.
- Under Which roles (none ticked means everyone), tick the roles you want, or none for all users.
- Choose the lists and tags to add everyone to (both optional).
- Tick These people have given me permission to email them only if it is true.
- Press Import users. When it ends you see "Done: N new, M already contacts, K rejected." Press Close.

The job also appears on the History tab.
What you get #
- The email comes from the account. The names come from the user profile; if both are empty, the display name is the first name.
- New contacts get the source
wp_users. They are Pending unless you ticked the permission box, in which case they are Subscribed and your name and the time are saved as the record of permission. Pending contacts are not emailed; see Double opt-in. - A user whose email is already a contact is not changed. They are linked to the user account and added to the lists and tags you chose, and count as "already contacts".
Warning: Do not tick the permission box for a whole site's users unless they agreed to marketing email. Pending is the safe choice.
Tip: To keep contacts in step with users, turn on Remove the contact when their WordPress user is deleted in Settings > Compliance (off by default). The contact then goes to the trash, where you can restore it for 30 days.
This is information, not legal advice.
Next: How to import contacts from a CSV file, Contact statuses.