Splunk provides a few built-in dashboards for that. Take a look in the Search-App under Status -> Search Activity.
Another option is to build some searches/reports based on the events in the _audit and/or _internal index. Like this one:
index=_audit NOT user="n/a" NOT user="splunk-system-user" earliest=-30d action="login attempt" info="succeeded" | bucket span=1d _time | dedup user,_time | stats count by user | sort -count
which displays on how many days the users have logged into Splunk in the last 30 days.
