There are plenty of blogs and articles that explain how to add properties to Advanced Search in MOSS, so I'm not going to get into that here.

What I have found is that there is an issue with the ModifiedBy property, where by default, the number of items found with this property is a big fat zero (0).

Finally, a solution that works!

The post is here: http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=2195994&SiteID=1&mode=1

I have copied Pritam Dahake's solution in case the forum post is somehow deleted:

Here are the steps to manually hook the Last Modified By field to the creawled field.

1) Open Central administration -> Shared Services -> SharedServices (X being the number of the shared services provider for the site you are creating a mapping)  - > search settings -> metadata property mappings
2) Click “Modifiedby” to edit the properties
3) Make sure  “Include values from all crawled properties mapped” is selected
4) Click “add mapping” and select the “Office” category from the dropdown list.
5) look for a property named “OFFICE:8” , click “OK”
6) Click “add mapping” and select the “ows_Last_x0020_Modified(text)” from the ALL categories dropdown list, Click “OK”
7)  Make sure the box is checked “Allow this property to be used in scopes”, click “OK”
8) Click on “Crawled Properties” link on the left side of the Shared Services Administration page.
9) Click On the “Office” Category.
10) The mapped property you just created should be listed in the “Mapped To” column of the Office:8(Text) category.
11) Click on the Office:8(Text) category to view the properties.
12) In the “name and information” section, you should see the “property Set ID:” value is  'F29F85E0-4FF9-1068-AB91-08002B27B3D9' ...This is very important!!!
13) In the “Mappings to managed properties” section the managed property you edited in steps 2 – 7 should be listed.
14) The box to “Include values for this property in the search index” should be checked.
15) Click “OK” to exit this page.
16) Open “Shared Services administration” -> “ShareservicesX” -> “Search Settings” -> “Content Sources and crawl schedules”
17) Click on “Local Office SharePoint Server sites” to expand the dropdown list and select “Start full crawl”
18) When the crawl has completed, navigate to the site collection -> click on the search tab.
19) Select “Advanced Search”
20) In the properties dropdown list select "Last Modified By"
21) Select contains
22) Enter a name that you know has added / edited a document
23) Click the magnifier to execute the search
24) Get back results.

 WOOHOO to Pritam!