On Centos 7.0, you are trying to resize using “resize2fs2 the new logical volume once you have add a new volume to your group;
[root@centos7 ~]# resize2fs "yourvolumegroup" resize2fs 1.42.9 (28-Dec-2013) resize2fs: Bad magic number in super-block while trying to open "yourvolumegroup"
Couldn't find valid filesystem superblock.
On Centos 7.x, its using now XFS (look into the /etc/fstab), default should be xfs.
Instead of using resize2fs, use this command: