Quoting from AAMAS2021 registration page:
While we strongly encourage workshop attendees to register for the whole conference (as the registration fees are quite low), we understand that some of you may wish to attend the workshops only. To accommodate this, the workshop-only attendance is free of charge. We are still working on the details for registering workshops only, and will publish them soon.
We join AAMAS board in recommending full registration, as it is as low as 60$ (USD)!