Here's the basic procedure:
1. Figure out which of the the users has the largest database. Then figure out where this user's data directory currently resides. You can find out where the program is looking for their data by opening Essential Forms and clicking on View>System Information. The System Information screen has a "Directories" section and on the third line under that is the entry for "Data". (Note that you may have to drag open the window to see the full pathname.)
2. After closing the program, move this person's Essential Forms data folder to a shared location on the server. Users will need read, write and modify rights to this folder. You can rename it if you like.
3. Open the user's Essential Forms back up, click on Configure>Data Directory, choose "Browse" and browse to the shared Essential Forms data folder on the server. Select it and click "OK", then "Use this directory". The program will close down, and when you reopen it, it should be looking at the data folder on the server. You can verify the program is now using the data folder on the server by looking at the System Information screen as detailed in 2. above.
4. (Here's where it gets tricky.) In order to merge the smaller database into the one that's now on your server, you'll have to open the program, open a client's tree by clicking on the plus sign to the left of it, right-click on the matter folder beneath it and choose "Export Matter(s)". If there is more than one matter under a given client, you can select multiple matters by using <Ctrl>-click or <Shift>-click, but the export function can only be used to export one client's matter(s) at a time. Export these matters to a portable storage medium or a shared network folder. Repeat for all clients on both of the computers containing the smaller databases.
5. Once all desired matters have been exported, you can repeat step 3 above for all remaining users or move on to 6.
6. Once all users are sharing the data folder on the server, you can import the matters from step 4 by opening Essential Forms, clicking on File>Import Matter(s). Luckily, unlike with an export, the import function allows you to select multiple matters for import, so you can use <Shift>-click to import all the matters you've exported to a given folder.
*Note that if the client name and the matter name of a matter being imported is an exact match for a client and matter already in the target data set, you'll see a dialog asking if you want to overwrite that matter. You can work around this by taking note of the client and matter names and saying "no" (don't overwrite). Then, you can find that client on the list, click on the plus sign, right-click on the matter name, and selecting "rename". You can just add a "1" or an asterisk or something to the matter name to slightly change it. Then re-import the matter that prompted the confirmation message. You can drag and drop forms from one matter to the other and then delete the superfluous matter.