The leader (or whomever bought the plugin) can locate the assets which have been installed to the engine directory (C:\Program Files\Epic Games\UE_4.19\Engine\Plugins\Marketplace).
Move the plugin folder from the directory above into the Plugins folder of your project (if you don’t have a ‘Plugins’ folder, create one. It should have a capital ‘P’ and sit in the root directory along with Binaries, Build, Content, Source etc. Remember to remove the original files (I do this through the Epic Launcher ‘remove local content’ option against the plugin, but just deleting will probable work too.
Depending on the plugin, as an additional step you may need to open your .Build.cs file and add the plugin as a Public Dependency. You may also want to right click on the uproject file and choose to generate visual studio / xocde files again.