Save Multiple Users into SharePoint List

SPFieldUserValueCollection pmCollection = new SPFieldUserValueCollection();

for...{
SPUser user= SPContext.Current.Site.RootWeb.AllUsers[your username];
SPFieldUserValue UserName = new SPFieldUserValue(web, user.ID, user.ID.ToString());
pmCollection.Add(UserName);
}

SPContext.Current.ListItem[your field name] = pmCollection;

// save item
SaveButton.SaveItem(SPContext.Current, false, string.Empty);
SPContext.Current.ListItem.Update();