Since the last releases of Blender, there is no more the possibility to copy/paste nodes between files (that was hyper-useful). Now the only solutions is to Group/Append Group. But I can't group AN nodes neither... so the question is not "how to group AN nodes", but "how to move them to another file", if there is another way to do that.
The only way I know of it's importing (appending in blender terms) the entire node-tree :/ File->Append->select the blend file to copy from-> click NodeTree-> click the tree of interest. Definitively not user friendly considering the more common case to import into your current tree, but you can invoke the subprograms inside the newly imported node-tree and use it in your main tree. Another way would be to add the code to recreate to the templates dir and repackage-reinstall the addon (or just add them directly in your addon's installation directory) for example:
[...] sourceObjectNode = self.newNode("an_DataInputNode", x = 0, y = 0, label = "Source Object") sourceObjectNode.assignedType = "Object" timeInfoNode = self.newNode("an_TimeInfoNode", x = 0, y = -200) [...]
See here for the full explanation of how to take advantage of multiple node-tree
Currently a simple to do that is to append the whole nodetree data from old file to new file, then copy wanted nodes (or delete unwanted ones) from it. Here is the way:
File > Append, then click on the old file, find NodeTree folder, then choose the nodetree datablock you want to append.
Tip: Relatively, you can completely remove an AN nodetree by hitting spacebar in Node Editor, then search for "Remove Animation Node Tree".