To transfer a large number of clients simultaneously, the most efficient method is to go the "Clients" section of your desktop app.

Once there, check the box at the top of the list.
This will select all the clients displayed in the list according to the filter you choose.
You can also select all the clients you want to transfer one by one.

Selection the transfer option, which you will find at the upper left of your screen.

Next, choose which groups or users you would like to send the clients to. Click "Transfer" to finalize the process.