You can use CloudTrail to see the dashboards that each user has viewed and then calculate an estimate of their usage cost based on that. I say “estimate” because it looks like CloudTrail tracks each view of a dashboard instead of sessions but billing is based on sessions.
This article shows how to use CloudTrail logs to create a dashboard.
You can use the same idea to create your own Usage dashboard to calculate an approximate usage cost per user. We use this method to calculate usage cost per user and per customer for our SaaS application. I don’t know if there’s a way to get more accurate numbers.