I assume this is basic functionality but I can’t find how to do it so … here I am.
I want to have a folder on Dropbox where I can drop documents from computers other than my main computer. Then I when I launch DT on my main computer I want these documents imported DT automatically, I shouldn’t have to do anything.
How do I do this?
Well, you will. 
DEVONthink is not intimately tied to the filesystem as the Finder is (and yes, there are sounds reasons why).
You can accomplish this through indexing but there is still some interaction needed to update the indexed group.
You can index via File > Index… or by Command-Option-drag and drop.
You can update by selecting the group and (1) clicking on it will update items directly inside it, (2) opening it via disclosure triangle or double-clicking, or (3) File > Update Indexed Itens…
Note: You should search the Forums for info on indexing, paying special attention to posts by Greg_Jones who makes almost exclusive use of it.
Sigh, another place where I have to work 
OK, I’ll read up on indexing. Thanks.
Of all the actions performed while working in DEVONthink, I find that thinking is the most difficult, and requires the most time. 
Haha! Nice, Bill. 