Profiles is good or you could also use logon triggers and create
yourself a little audit type table that is written to as users logon and logoff the database (if that is what you are trying to do)

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LOGON and LOGOFF triggers can be associated with the database or with a schema. Their attributes include the system event and username, and they can specify simple conditions on USERID and USERNAME.

LOGON triggers fire after a successful logon of a user.

LOGOFF triggers fire at the start of a user logoff.

Gregg