Is there a way to increase the thickness of all lines in a group?

This might work for scaling many strokes varying in width by a percentage. Use Scale with Scale Strokes & Effects checked and the desired percentage, e.g., 50%. Then reverse the scaling with, e.g., 200% but with Scale with Scale Strokes & Effects unchecked.

Oh!! The scripts posted by Kurt Gold work in groups! 
Thank you, thank you, thank you! I will mark that answer as the right one. Appreciate everyone's help!

Okay, I know I'm being slow on this. Just got a major project off and now have a little more time to deal with this. So I tried the script that I posted earlier (From Ray Craighead) and I can't get it to work. 
I pasted the code into Notepad, saved it with a .jsx extension, put it into the scripts folder in Illustrator. When I have my Illustrator file that I want to work on open, I select the group that I want to make bolder (thicken the lines) and go to File/Scripts. The script is there, but when I click it and the dialogue box pops up and I enter a decimal and hit okay or return, it does nothing. The line thicknesses do not change and no step is added to the history panel. 

Thoughts for me on what I'm doing wrong?

Both scripts shown by @Kurt Gold  do what I think @Lynnette7 wants.

The scripts also work in groups and with compound paths.

Further:
All scripts shown here in the thread so far work exclusively with point as a unit. (But this could easily be changed).

Two questions remain:

1. What unit of measurement does Lynnette7 use for the stroke width?
2. Does Lynnette7 want to change the thickness once in the scripts shown by Kurt or does she prefer the input option via @rcraighead's dialogue box?

There are two simple, about twenty years old scripts by Muchacho (stroke_plus and stroke_minus) that increase or decrease all selected strokes by 0.5 pt.

They still work in current versions of Illustrator. Increase/Decrease values can be modified in line 5 and 6.

Download Stroke Plus/Minus

Oh, I found this, which I think is what @Ton Frederiks was talking about. The thread is here: https://community.adobe.com/t5/illustrator-discussions/how-to-apply-a-brush-and-keep-stroke-weights/m-p/10976459

And the specific message I'm referring to contains this...
_______

This should work. Make sure to enter the percentage with a decimal point.

//by Ray Craighead
//Resize selected path stroke width(s) by percentage.

function scalePathWidth(){
    var aDoc = app.activeDocument;
    var sel = app.selection;
    if (sel.length < 1){
        alert ('Make a selection of paths and try again');
        return;
        }
    else
    {
        var myInput;
        userPrompt();
        var myPercent;
        function userPrompt(){
            myInput = prompt('Scale Percentage (0.1 = 100%)', .100);
            myPercent = Number(myInput);
            }
        for (var i = 0; i < sel.length; i++) {
                var myWidth = sel[i].strokeWidth;
                sel[i].strokeWidth = myWidth * myPercent;
                }
            }
        }

scalePathWidth();

And here's a .jsx file you can download and install:
https://www.dropbox.com/s/yjg7jrj7wkak5p8/scalePathWidth.jsx?dl=0 Edit: I noticed I left an unused variable so I updated the script.
_______

The Dropbox link no longer works. So I wondered if anyone smarter at coding than me knows what unused variable needs to be fixed in the script?

I have a script by @rcraighead that resizes selected paths by percentage, but I cannot find it anymore on this forum: scalePathWidth.jsx

I don't want to post it without his permission.

@Lynnette7 

1. You have different groups of paths with different line thicknesses in a layer - right?
2. All paths (of a group) should become thicker by the same value, in addition to the existing line thickness?
3. All strokes have the same colour?

Try this (sorry for my German UI):

select your group

Add menu: Effekt -> Path -> Outline Stroke

Nothing happens - only the small circle to the right of the group name is filled now

Create a new layer and drag the whole group onto this new layer

and click on the circle with a red border (in the screenshot) to the right of the name of the layer.

Add a new stroke in the Appearance Panel

Set the stroke width as desired, but double the width as the new stroke will automatically be aligned in the middle. (I have coloured the contour yellow so that it is easier to see.)

Done!

You can select based on depth using the Layers panel.

The top most layers are the closest.

Shift-click the gap between the circle and the end of the panel (blue square) to select.

Once you have your depth selected you can adjust the stroke or Group them as needed.