Add Marcel to pyhep activities (#1706)

* Update pyhep.md

* Update pyhep.md

---------

Co-authored-by: Eduardo Rodrigues <[email protected]>

Author: riga

_activities/pyhep.md

@@ -14,6 +14,7 @@ the sharing of knowledge and expertise. It embraces the broad community, from HE
 * Ianna Osborne (CMS and IRIS-HEP, Princeton)
 * Matthew Feickert (ATLAS and IRIS-HEP, University of Wisconsin-Madison)
 * Nikolai Krug (Belle II, LMU Munich)
+* Marcel Rieger (CMS, Hamburg University)
 
 All coordinators can be reached at <[email protected]>.