Hi @concejaa!
That's totally possible and common practice.
The actual way of onboarding your partners depends on your current authentication approach.
If you are
- Manually managing users in AEM, you can just create new users for your external partners.
(I wouldn't recommend manually managing users in AEM, though.) - Using any kind of Single Sign On (SSO) system, you need to create users for your agency in that system or the source of that system (probably an identity provider, short IDP).
- Using Adobes Identity Management System (IMS), you can manage these users through Adobes Admin Console, see this documentation.
When onboarding additional users, especially external ones, please make sure to have a proper authorization concept in place that allows you to restrict a users access based on group memberships to certain areas of the system/content.
Also, depending on your setup, you may need to check your infrastructure to allow external access to your AEM authoring instance. If the authoring instance is publicly available on the internet, that's not an issue. If it's hosted on an internal network or otherwise access restricted (IP allow lists or similar), you may need to make adjustments to allow for external access by your agencies.
Hope that helps!