How to add existing log files to a newly created filegroup ?

Transaction log files are not part of a filegroup, only MDF and NDFs.  Transaction log is managed separately.

You should properly remove logfile2.ldf instead of trying to add it to a filegroup.

• Check log file usage
  DBCC LOGINFO
  If there are active VLFs in logfile2, means it is still in use
  
  
• Make logfile1 the active (primary) one and force SQL Server to stop using the logfile2
  (adapt the size and filesize if needed)
  USE [YourDatabase];
  GO
  ALTER DATABASE [YourDatabase] MODIFY FILE (NAME = 'logfile1', SIZE = 100MB, MAXSIZE = UNLIMITED, FILEGROWTH = 50MB);
  GO
  
  
• Shrink logfile2
  DBCC SHRINKFILE ('logfile2', EMPTYFILE);
  
  
• Force SQL Server to switch log files to logfile1
  CHECKPOINT GO BACKUP LOG [YourDatabase] TO DISK = 'NUL';
  GO
  DBCC SHRINKFILE ('logfile2', EMPTYFILE);
  
  
• Try to remove logfile2
  ALTER DATABASE [YourDatabase] REMOVE FILE logfile2;

Make sure you have a full database and log backup before making these changes!