You don't need additional modules to introduce data from referenced entities into your views, you can do it with Relationships.
Your question title is about the Views field view module, but your question text reveals what you're really after, so I won't go into the Views field module solution because it's not the best way to go (performance-wise) if you can do it otherwise.
The Entityreference field on your "Webinar" nodes only lists titles of the "Speaker" entity, both on node pages and in Views.
To display information other than Title of the "Speaker" entities within "Webinar" views you have to add a Relationship through the entityreference field. That acts as a bridge to the other tables/data and makes them available for use within views.
In your case you should:
- Add a Title field and the "Speaker" entityreference field for your "Webinar" nodes (you probably already have this step)
Edit the Speaker field and under
Multiple field settings untick
Display all values in the same row.
This will make every speaker display as a separate views row.
- Add a Relationship Content referenced from field_speakers (or whatever the field name is in your case), require it if you don't want to see webinars in your results that don't have any speakers selected
- Add the "Speaker title" field, it will automatically use the above Relationship
- Add the "Speaker company" field, it will automatically use the above Relationship
Format Settings set the Grouping field Nr.1 to Title (the Webinar one).
This will take care of the way you want results grouped under each webinar title.
- Edit the Title field (Webinar) and set it to Exclude from display and under
Style settings set the Customize field and label wrapper HTML to H2 or whatever makes the most sense in your case.