With Workgroup Manager's departure in OS X Server for Yosemite, administrators are left without a GUI-based way to export users and groups out of Open Directory. While Workgroup Manager contained funcitons for both importing and exporting records, the Server app only has the ability to import records from properly formatted files (like the files that Workgroup Manager would generate). Thankfully, we have a full suite of "ds" commands on the command line to generate the files for us.
For Users:
dsexport /Users/Shared/exportedUserRecords.out /LDAPv3/127.0.0.1 dsRecTypeStandard:Users
For Groups:
dsexport /Users/Shared/exportedGroupRecords.out /LDAPv3/127.0.0.1 dsRecTypeStandard:Groups
Of course, you can take any record type and do exports using this method and the proper "dsRecTypeStandard:" definition. If you prefer the command line, dsimport can handle your imports for you, rather than the GUI of Server app.