In order to retreive the role of sales rep from employee record for a transaction search, the exact join would be difficult to get. Try like {salesrep.role} or {salesrep.roleid} . This may or maynot be as in your NetSuite. Such fields are not available under the drop down list .
Sample:
Under results tab of the transaction search:
Formula(Text) = {salesrep.role} would return the role / roles of the sales rep from employee record.