If your sales team is working in salesforce, this can be acheived with a salesforce campaign.
- create a campaign for the event in SF and create campaign status for Registered and Attended
- import seperate lists into hubspot for Registered and Attended
- use hubspot workflow to send attended to this SF campaign with the campaign status Attended
use another hubspot workflow to send registered to same SF campaign with the campaign status registered
- your sales team can use lead views and contact views to see the prospects assigned to them and their campaign status