How to add a user and multi users to the People picker field in a sharepoint list programmtically

Coding:

  if (spProjectListItem["USER"] != null)
      projectMemberValue = spProjectListItem["USER"].ToString();
      SPFieldUserValueCollection oFieldUserValueCollection = new SPFieldUserValueCollection(spDLRWeb,
                                                                                                                    spProjectListItem["ProjectMember"].ToString());
       spListItem["USER"] = oFieldUserValueCollection;