Depends on what exactly you want them to do. To just see the library, the group library's URL will do fine. It's what you see when you view the library online, i.e.
https://www.zotero.org/groups/[libraryID]/items
where libraryID is some 5-6 digit number.
Or do you want them to join the library? If so, if it's an open library they can just join at
https://www.zotero.org/groups/[libraryID]/
If it's a public closed library, you'll need to send them invitations at
https://www.zotero.org/groups/[libraryID]/settings/members
https://www.zotero.org/groups/[libraryID]/items
where libraryID is some 5-6 digit number.
Or do you want them to join the library? If so, if it's an open library they can just join at
https://www.zotero.org/groups/[libraryID]/
If it's a public closed library, you'll need to send them invitations at
https://www.zotero.org/groups/[libraryID]/settings/members
Where do I find my library ID?
Merci.