This functions works differently depending on whether you are running Jira 6.4 or above, or not. From Jira 6.4 the worklog is indexed and is available for searching.
If you are on 6.4+ this takes the same arguments as
commented, you can also use inRole, inGroup, and roleLevel predicates, to eg search for work logged by a user with a particular role. As above, roleLevel is for filtering on the security role level of the worklog.
If you are on prior to 6.4, you cannot use role, group and updatedby, which aren’t supported for searching worklogs.
Also, prior to 6.4 the following query would return incorrect results:
issueFunction in workLogged(on "2015/07/28" by admin)
It would return issues where work had been logged by admin, and work had been logged on that date, but not necessarily by that user.
To find all work logs by members of the Developers role in the preceding calendar month:
issueFunction in worklogged("after startOfMonth(-1) before endOfMonth(-1) inRole Developers")
As of 6.4 you can do much of this in plain JQL, but you don’t have the role or group functions available.