of certain offenses.
Dietl/Lorenze gives "joint plaintiff" as a translation, but this appelation doesn't wotk very well in US English, because we really don't have such a thing in the US. A "Nebenklage" is translated as civil action incidental to a criminal action. Again we don't have such a thing.
Your best bet may be "Joining Complainant (or Plaintiff) followed by the answer I have given (in parentheses).

ref.: Langenscheidt!-to my surprise. This does seem to be a term used in the US.

.. the NYCCELP members and supporters in picture are (left to right): Sam Hirsch (Greater New York Labor-Religion Coalition, co-plaintiff organization), Glenn ...
www.nmic.org/nyccelp/news/ october%2011,%202000%20-%20judge%20strikes%20down%20ll%2038.htm - 4k

. The plaintiff's claim against Scipper does not preclude realignment since it may be construed as a cross-claim against a co-plaintiff pursuant to Rule 13(g) of ...
www.olemiss.edu/~llibcoll/ndms/june96/96D0089P.html - 10k

AAP a Co-Plaintiff in South Carolina Internet Law Challenge. AAP is a co-plaintiff in another state Internet harmful to minors law challenge. ...
www.publishers.org/press/monthly.cfm - 32k

--------------------------------------------------
Note added at 2003-01-28 04:27:27 (GMT)
--------------------------------------------------

452.300]. NAME USED TO DESCRIBE SPOUSE FILING FOR DISSOLUTION OF MARRIAGE:
Plaintiff [or Co-Plaintiff if the petition is filed jointly. ...
www.divorcesupport.com/statelaws/missouri.shtml - 19k

. 94-5016, Chapter 7. ISSUE: Whether co-plaintiff-debtor was bound by employment terms agreed to by plaintiff-trustee and the plaintiffs\' attorney and approved ...
www.sdb.uscourts.gov/Decisions/1996Decisions/ 1996%20Decision%20Summaries.htm - 25k - In cache - Paginas similar

Eviction Sample
... 1520 ORLANDO Plaintiff Property Phone SABAL PALM AT METROWEST APTS SABAL PALM AT METROWEST APTS 407-523-0088 Co-Defendant Co-Plaintiff Phone GIALLOMBARDO MYRNA ...
www.asapscreening.com/AsapFiles/evictsmp.htm - 16k -

http://www.law.cornell.edu/ny/ctap/085_0413.htm

Someone mentioned usage in the U.S. I'm not aware of any criminal proceedings which refer to a "plaintiff." That terminology is reserved to my knowledge to civil cases.

im Strafrecht/in penal law;

or 'intervener'