peoplePick1.CommaSeparatedAccounts = SPContext.Current.Web.CurrentUser.Name;

SPListItem item = listItems.Add();
... + übrige notwendige listitem-FeldZuweisungen
string[] UsersSeperated = peoplePick1.CommaSeparatedAccounts.Split(',');
SPFieldUserValueCollection UserCollection = new SPFieldUserValueCollection();
foreach (string UserSeperated in UsersSeperated)
{
    myweb.EnsureUser(UserSeperated);
    SPUser User = mySite.SiteUsers[UserSeperated];
    SPFieldUserValue UserName = new SPFieldUserValue(myweb, User.ID, User.LoginName);
    UserCollection.Add(UserName);
}
item["Mitarbeiter"] = UserCollection;
item.Update();