Need simple black, thin line to create borders

Pat, there is more than one way to accomplish what you want.

Assuming the 3 photo groups are individual files (not yet merged with the background file):

1. Open your group file.
2. In the Layers panel, click on the "Create a new layer" icon. (Or you can go to Layer>New>Layer.)
3. Select>All (Ctrl-/Cmmd-A). You will see marching ants around your group file.
4. Go to Edit>Stroke (Outline) Selection. In the dialog box, enter the stroke width and color of your choice. For Location select Inside.
5. Select>Deselect (Ctrl-/Cmmd-D).

NOTE:

The advantage of having the stroke on its own layer is that it gives you more flexibility in editing. You can change its color, width; apply a bevel, texture, etc. If you decide you don't want the stroke, just delete this layer.

Assuming the 3 photo groups are already merged with the background file:

1. Make a selection of the group using the Rectangular Marquee tool (M).
2. Place your selection on its own layer (Ctrl-/Cmmd-J).
3. Follow Steps 2 through 5 as delineated above.

Please try this,

Add a new blank Layer at the very top.

Select the rectangular marquee tool.

Mate a selection around the photos you want to group.

Go to Edit > Stroke select black for the color, and try a width of 10 px - if that isn't correct press ctrl/cmd+z to undo and try it again with a larger or smaller width.

Duplicate that layer (ctrl/cmd+j)

Press ctrl+cmd+t to transform us the handles on the bounding box to move it around the next group.

Duplicate that layer (ctrl/cmd+j)

Press ctrl+cmd+t to transform us the handles on the bounding box to move it around the next group.

Frizzylee - there are always more than one way to do something in PE so just use the method you feel most comfortable with (or can understand!). Fotofrank's method is I think the easiest to understand and will take you only a few minutes to do. Basically you draw the box round the first group of pics using the rectangular marquee tool to enclose them (on a separate layer) and using edit/stroke outline to draw the lines. You then duplicate that layer and using the move tool, move the box over another set of pics and adjust to fit, then do the same for the third set.
Good luck.

In your 3 screen shots above, for each of them use the rectangular marquee tool to draw a box of marching ants round your grouped images. Then do the stroke outline etc. Then you can amalgamate your 3 sets of boxed images onto your big sheet. 

FWIW, it sure looks like the method suggested by fotofrank is by far the easiest and quickest way to accomplish your goal.

To Tpgettys. Hi Tom, You are absolutely right that fotofrank has the quickest and probably easiest method. I saw that immediately when it popped up. However, (and I make no excuse, it's just the way I am) I want what I want when it comes to media, meaning video and photobooks. I make no claim for excellence but when something isn't what I envision I can't quit until it is. It's hard and there have been some miserable times, but if I stick to it long enough and don't give up, I am always, always glad I did. When I made one of my last videos, there was a little blue train on a birth announcement. A friend said, "You ought to make it so the little blue train comes off the announcement and leads into the next segment." Well, it nearly killed me but by gosh that little blue train did just what she suggested. And I have never ever been sorry.

Forgive my rattling on, but I am taking a breather here. What I want is for my groups of photos to be grouped together in a rounded off rectangle pretty much like Sepiana's are when she quotes from what someone wrote. Rounded off rectangles, that's what I want. Further, I plan to use it now and then in a photo book so I want those pages to reflect one another style-wise. The squared off approach accomplishes the same thing, but it isn't the same.

I will keep at it, but right now I have yet to get it done.

Thank you for your observation. You are not wrong.