Red nodes aren't affecting result I am creating a lens flare for my scene using the node editor. 

I finished setting up all the nodes and had a final render which looks ok. However, when I tried to group it (CtrlG), all the nodes being grouped turned red.

It's my first time trying to group nodes so I'm not sure if it's normal that all the nodes turned red. Moreover all the nodes that is under the group doesn't work when being rendered! the nodes no longer work after I grouped them. How could I fix this? 

Here's my blend file.
Those nodes are red because they are muted, which is a means of disabling a node temporarily. M toggles the selected nodes muted/unmuted.

Grouping in your file works fine for me, so I'm guessing you pressed M accidentally while pressing CtrlG. 

To fix, just select the muted nodes and press M again.