I’ve been away from this forum for a few months, so sorry if this topic has been covered and I missed it.
I have probably two hundred groups. I realize this isn’t the DT way of doing things, but organizing things yourself is a hard habit to break. I associate things in ways that I would never expect DT to always get.
Here’s the problem: sometimes when I click the “Classify” button, the group I want isn’t one of the ones suggested. So now I have to drag the item over a long list looking for the right group to drop it into. So I’d like to make a short-term and a long term suggestion to deal with the situation.
Short term suggestion: in the Classify drawer, make a field where you can just type the name of the group you want to move the item to.
Long- term suggestion: In each document in the DT database, make a box, maybe somewhere up in the toolbar, where you can type the name of the group you want to “force” the item to belong to. Maybe there could be a dropdown box of ways you’ve tagged things before. Actually then, I’d never have to fool with groups at all. I could just do a search for all the things I’ve tagged a certain way any time I want to see them together.
I use “control click” on the item and use the “Move To” submenu - I certainly find it easier than dragging the wee beastie to it’s lair. 
Re your short-term suggestion: Rather than dragging and dropping, I find it much quicker to control-cllick the document name (which is already selected, e.g. in the Vertical Split view, left column) and choose “Move To.” Choose your target group and the document is immediately placed there. There are similar procedures to allow you to replicate the document to another group (which I find very useful), or to duplicate it to another group.
Re the long-term suggestion: You could use the procedure noted above to force locate the document. In a sense, your request has already been met.
But you are on to something a bit more, as you want to add a metadata field to each document and be able to search for that metadata – i.e., you want to add searchable metadata to the document. Christian has indicated that there will be future developments in this respect. There are very powerful potentials in such approaches. I’m in complete agreement with you in looking for such added powers.
Right now, it’s not immediately obvious that you can search for items by group. But you could approximate this by setting Tool Search options to search in a designated group, then search for “the” – or do a wild card search for the letter “e”. An approach like that will display (with high probability) all the contents of the designated group. 
I get a kick out of looking for tricks to milk as many logical possibilities as I can out of DT’s current Search options. Noted above is a way to search for documents by group. I can do a multiple step operation to emulate the NOT operation. But there’s no way I can see to handle NEAR or BEFORE types of Boolean searches. We can do much of that in DEVON agent, but not yet in DEVONthink. (Christian has indicated that more Boolean operators with come to DEVONthink in the future.)
Hope this helps.
Thanks to both of you! I didn’t know about the control click before. It’s certainly easier. I’ll have to try it for replicating and duplicating too. And I look forward to the metadata developments Bill mentioned.
Once again, thanks for the very useful tip.
Actually, it might be useful to be able to classify from the keyboard without needing to use the mouse. I think it would be really convenient if there was a way to enter a classification group in a text entry field, particularly if the classification was prefix matched as you typed. In other words, something like being able to enter:
command-C => brings up classification dialogue box
engi => brings up a list of all groups with names begining with “engi”, kind of like a spotlight search
=> moves or replicates the current doc in the selected group(s)