Do organizations have to utilize the "My Room for Scheduled Meetings" for Invitation Only events?

This is always optional. Organizations can still interact with candidates in their Group Room on Event Day. From there, they will have the option to conduct panels or give presentations. They can also engage with candidates 1-on-1 using audio, video, or text chat. 

It is not required to set up pre-scheduled 1-on-1 meetings with candidates.