At the moment we don't support directly hiding projects while allowing participation. We will soon support sharing public links to draft projects for easier collaboration, but in the meantime there is a workaround.
You can create a Draft folder, and within the folder a Published project. You'll be able to share the project, but because its in the folder it will not be visible to the general public. Some considerations:
Warning:
Emails will be triggered for users following a topic or an area, if the project is tagged with those. Don't tag the project if you don't want these emails to be sent.
If someone knows the title of project it can still be searched in the header navigation.
Events will be visible in the homepage anyway