Network Share set permissions don't work
To create a properly working Projects share do the following:

On your external disk create a folder "Projects". Share this Projects with default permissions in Server.app > File Sharing. The default permissions are 755, the default owner is the user who created the folder and his primary group (e.g. adminuser:admin):

drwxr-xr-x+   6 adminuser  admin  -       204 Aug 23 10:54 Projects
 0: user:_spotlight inherited allow list,search,file_inherit,directory_inherit


In this folder create another folder with the name Projects. Change the permissions to 770 and the group to staff:

mkdir Projects
chown adminuser:staff Projects
chmod 770 Projects


The permissions now look like this:

drwxrwx---+   6 adminuser  staff  -       204 Aug 23 10:54 Projects
 0: user:_spotlight inherited allow list,search,file_inherit,directory_inherit


Then add an ACL for the group staff:

chmod +a "group:staff allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit" Projects


with the result:

drwxrwx---+ 2 adminuser  staff  -   68 Aug 23 11:09 Projects
 0: group:staff allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit
 1: user:_spotlight inherited allow list,search,file_inherit,directory_inherit


The complete path to the Projects folder (containing a test folder and a test.txt file - Data is the name of my volume on the external disk) is then:

/Volumes/Data/Projects/Projects:
total 0
drwxrwx---+ 4 adminuser  staff  - 136 Aug 23 11:24 . #this is /Volumes/Data/Projects/Projects
 0: group:staff allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit
 1: user:_spotlight inherited allow list,search,file_inherit,directory_inherit
drwxr-xr-x+ 6 adminuser  admin  - 204 Aug 23 11:09 .. #this is /Volumes/Data/Projects
 0: user:_spotlight inherited allow list,search,file_inherit,directory_inherit
drwxr-xr-x+ 2 adminuser  staff  -  68 Aug 23 11:24 test
 0: group:staff inherited allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit
 1: user:_spotlight inherited allow list,search,file_inherit,directory_inherit
-rw-r--r--+ 1 adminuser  staff  -   0 Aug 23 11:24 test.txt
 0: group:staff inherited allow read,write,execute,append,readattr,writeattr,readextattr,writeextattr,readsecurity
 1: user:_spotlight inherited allow read,execute


Now any subsequent file and folder of /Volumes/Data/Projects/Projects can be modified by any user in the group staff. You may tweak/tighten some of the permissions by not adding them (e.g. readextattr and writeextattr)