Added as a "user" to another account, but I don't see the webinars for that account.

Hi @arosearose by your colleague adding you as a "user", meaning you are a user under the owner of the account; therefore, you can be assigned licenses, and acquire Account settings based on whats set by an Admin or Owner on the account. 

However, as to your point, what are you referring to as managing the webinar? Helping the host with the webinar when live or help setting up the webinar managing, settings, etc. for the colleague? 

Hoping that your webinar went well! @arosearose 

For Webinars you are free to be added as an alt-host to help start and run webinars alongside your colleague.
In addition Zoom easily allows teammates to share Scheduling Privileges with one another, this would allow you to view, edit, schedule webinars on that other persons account.

Finally, Zoom Events offers an entirely different collaboration model with a Hub Owner and up to 4 additional Hub Hosts who can all create, manage, edit, publish events (single session or multi-day event) might be worth checking out.