You can check the Azure audit logs but they only go back 30 days so unless its within that time or you offload the logs for long term keeping there isnt much you can do unless you are also using some auditing on-prem as well.
Information about user joining a group

Oscar da Assuncao Alfredo Filho
21
Reputation points
Hi!
We have in our company an Active Directory connected with our Azure AD via AD Connect and this is working perfectly.
We don't know how to find audit information to know exactly when (date) a user was assigned to a certain group.
Can you tell me if this information is enabled or do we have to enable it? Can you help us?