For version 14:

This works AFTER you have imported all of your Contacts, if you need help with that, please see https://www.odoo.com/slides/slide/contacts-and-importing-data-609 and https://www.odoo.com/documentation/14.0/applications/general/export_import_data.html

1. Export all of the Contact records representing who you intend to invite (Name, Email, ID field).

2. Update that export by duplicating the Email field (call it Login), adding a "Groups" field and filling in all rows as shown below, and renaming ID to "Related Partner/Database ID" so you have something like:

"Name","Login","Email","Related Partner/Database ID","Groups"
"Ray Carnes","customer@company.com","customer@company.com",10,"User types / Portal"

3. Import this file:

4. After the import, select all newly imported record and from the Action Menu, select SEND PASSWORD RESET INSTRUCTIONS

Test this with a single user in the upload CSV, to make sure you are happy with the Email (which you can edit from the Email Templates Menu).

Assuming v9.0 Enterprise. Yes it's possible with export-import.

- Export the names, email addresses, and IDs of the customers you want to give portal access to.
- Export one or two user records to see how it's structured. Make sure you export the "Portal" checkbox and the "Related partner". This will be your template for the next step.
- Use that template and list all your customers as rows. Make sure the "Portal" checkbox is True. Make sure that the "Related Partner" field points to the customer ID you have exported in the first step. Mass import this csv file.

Notes: This will send a "Create your account" email to all these portal usres (where portal users need to create their password). Test this in a test instance where you disable your outgoing email servers. Test this with a very small number of customers at first. This will require that you learn about import in Odoo; the faq on the Import page in the application is a good start.