def select_by_users(df,users):
    """Selects some of the users so we do not
    need to process every user in the group
    
    Parameters:
    -----------
    df : pandas dataframe
        Dataframe of all messages 
    users: string array
        The users that you want to keep
        
    Returns:
    --------
    df : pandas dataframe
        Dataframe of all messages with selected users
    
    """
    df_new =  df.loc[df['User'].isin(users)]
    return df_new