проводить пленарные заседания(e.g.: Periodically, the Court sits in en banc sessions (in which the entire court participates), commonly three to six times each year. Alex_Odeychuk)
замещать должность(sit in for somebody: to do a job, go to a meeting etc instead of the person who usually does it: This is Alan James sitting in for Suzy Williams on the mid-morning show. LDCE Alexander Demidov)