How to place subgroups of pages inside other subgroups
While our system doesn't have built-in support for organizing page groups *within* other page groups, there is a workaround that allows you to fake the effect of having a thumbnail grid pages inside a drop-down menu.
The process involves using two separate page groups (one drop-down menu and one thumbnail grid), hiding the thumbnail grid, then linking to it from the drop-down menu:
If you don't already have a thumbnail grid collection of pages setup, create one: https://help.22slides.com/frequently-asked-questions/how-to-create-a-collection-of-other-pages
Set your thumbnail grid page to be hidden, using the "eye" icon to the right of your page's name.
If you don't already have a drop-down menu setup, create one: https://help.22slides.com/frequently-asked-questions/how-to-create-a-drop-down-menu
Create a new page, and choose the "Link" option.
Click on the new link page you just created, give it the same name as your thumbnail grid page you created earlier.
In the "destination url" field, enter the URL to that same thumbnail grid page, starting after the domain name and including the first forward slash (/).
If you need help finding the URL, click on your hidden thumbnail grid page, then click the link that's in orange at the top of the page.
Copy only the forward slash and the page name. In this example, it's "/events". Then paste it into the "destination url" field.
Now drag-and-drop your new link page into your drop-down menu. When visitors click the link page inside the drop-drown menu, they'll be pointed to your thumbnail grid page.