First you'll need to add a Group. Do so via the Groups button on the Library tab.
Once it's added, a new column with the group's name will appear in the library tree. Put a tick next to each file/folder you want to share with the user(s). If you don't want to share the files/folders publically, untick the Public box.
If they're not already there, you'll need to add the contact you want to share to with to your contacts list. Probably the easiest way to do that is have them send their contact link to you (Contacts > username menu > Copy Link), and for you to open that link (right-click on the Library button, Open Links, paste link, OK, add to contacts).
Finally associate the contact to your new group (Open the Contacts panel, right click on the contact, Groups, tick appropriate group).
At this point that contact and any other contacts linked to the group will be able to search or browse the files and folders associated with that group.
This might be handy: https://support.fopnu.com/groups
Hope that helps.