Q: How do I label a photo in the iphoto program

Nowhere can I find info on how to put a label into a photo like I could do with older versions.

What kind of label?  Do you want to add a title or description to the metadata, or do you want to write text directly into the photos?

I am running El Capitan and the newest iphoto program.

You must mean Photos 1.3.  Select the  photo and type Command + i to bring up the Info pane for that image.  There you can add Title and Description to the image:

There must be apps for putting labels into photos.  Such a s a group photo with 6 people in it...

Did you have a question? There are indeed many such apps, depending on exactly what you mean by "label".

You can use Photo Editing extensions from Photos to write into the image, if that is what you want to do,  but the watermark tools extensions  or text tools are still not versatile enough.  But see this list of extensions:

  Photo Editing Extensions for Photos for Mac

Most promising is the External Editors extension. It lets you call Photoshop or other image editors, like Preview.

See:   External Editors in Photos Are Here

I want to add little boxes (better than balloons but I'd accept that) over the six people in the photo with their names.

So you essentially want to deface the photo? You can do this with Preview.

Oh that was great fun!  Thanks!!