Login to customer tenancy as a partner

#1

We have a portal which is used by customers, which is integrated with Okta.
We also have partners, who would create customer accounts and manage the customer’s account in the portal on the customer’s behalf.
A partner can manage thousand’s of customer account
What is the best approach to enable partners to login to customer’s accounts?
Is there a way to implement this without adding a partner user to the customer’s group?

#2

Some improvements announced for this model: https://www.okta.com/resources/oktane-content/protect-against-breaches/#roadmap-experiences