From any of your communities, click Activity Library in the side nav.
This will bring up premium units, and also your own library of units.
Click the three dots of any unit to add it to your personal library of units.
Next, click “Save a Copy!" to confirm the addition to your library.
Pick the community where you want the unit to appear and click on "Yes, copy it"
Go to "My Library" to view your added units in your library.