Welcome!

simpler variant, directly from the contact, that:
​# Create the portal wizard (it auto-populates user_ids from selected contacts = records)
​# Keep only wizard users who do NOT already have a portal user linked
​# Grant portal access to each eligible wizard user

portal_wizard = env['portal.wizard'].create({})
for user in portal_wizard.user_ids.filtered(lambda w: not w.partner_id.user_ids.filtered(lambda u: u.has_group('base.group_portal'))):
    user.action_grant_access()

• from contact:
• • Select contacts and grant portal access by sending the “Portal: User Invite” email
• efficient bulk solution
• • export contacts ID (not External ID) and email address of desired contacts and edit:
  • • rename columns ID to “related partner/database id"
    • rename email column to “login”
    • add column “groups_id” with value “User types / Portal”
  • go to users and import the file
  • select the imported users and send password reset instructions
• to send a personalised email:
• • adapt the template "Settings: New Portal Signup"
  • create a server action to send the personalised email:
• 

records.with_context(create_user=True).action_reset_password()

• If desired, to use/adapt other templates like “Settings: New User Invite” 
• • Adapt the template and also replace in all languages <a t-attf-href=“/web/login?auth_login={{object.email}}” with <a t-attf-href=“#{object.signup_url}”
  • adapt server action to send the “Settings: New User Invite” email

template = env.ref('auth_signup.mail_template_user_signup_account_created')
for record in records:
    template.send_mail(record.id, force_send=True)