When using the trigger "User added to group" on a Automation for Confluence rule, the only available smart value is userAdded and group.
The userAdded returns the Display Name of the user.

It would be very useful if it returns the user object as per : Smart values in Confluence automation

User properties
user – Returns all available user properties
user.fullName
user.publicName
user.emailAddress
user.timeZone
user.accountId