Unless you were set as the Alternative Host, you may not be able to open the meeting. It really depends on how they set the meeting and if Join Before host is enabled.
If they are able to login and set you as an alternative host (Assuming you are also licensed on the same account), then you would be able to start the meeting and take host controls in their place.
Hope that helps and please make sure to mark the solution as accepted if this information is what you needed.