Bookmark collections are only visible to the user groups they've been associated with.
Consequentially, if you created a bookmark collection but didn't associate any of your user groups, no one (yourself included) will be able to see it from the browser's new tab.
When you associate a bookmark collection to certain user groups, all the members of the user groups will be able to see the bookmark collection and select them as their default in the browser's new tab.
To associate a bookmark collection to user groups, click on the groups and go to settings.
From there, in the "Bookmark collection" dropdown, select the collections you want to be visible to the members of the groups.
When done, hit save.