You can't do what you've asked, but there are three alternatives I'll describe here

1. Include a syntactically valid smb.conf snippet that defines your users.

    valid users = user1, user2, user3
   

   Include this snippet in your share definition. For example, if the snippet is called /usr/local/etc/smb.valid_users.conf you would declare your share like this

    [example share]
    path = /example/path/here
    read only = no
    guest ok = no
    include = /usr/local/etc/smb.valid_users.conf
   

   When you change the list of users you may need to restart samba (systemctl restart smbd). Notice that because this is an include, any valid code can be added - including entire shares that give away far more access than you would expect. Do not allow untrusted users access to change the configuration file!

2. Put the users into a group and reference the group in the share definition.

    valid users = @usersgroup
   

   Then exactly as usual you create the group usersgroup (see groupadd or addgroup depending on your OS variant) and add your users to it

3. Template the smb.conf and substitute into it as necessary. (The least favourite of my options.) Copy smb.conf to smb.conf.template and use this line in your share definition,

    valid users = %USERSLIST%
   

   Then use something like sed to substitute into the template

    sed 's/%USERSLIST%/user1, user2, user3/g' /etc/samba/smb.conf.template >/etc/samba/smb.conf
   

Just add comma ',' if you want multiple valid users. See line that I used below.

valid users = @group1, @group2

Each samba user must have a normal linux account as well.

1. Make sure that every user can access the common media folder on the unix side (without samba); alternatively, you can set force user in smb.conf
2. Make sure each user has a samba password set. You can set it with sudo smbpasswd -a your_user
3. Look at /etc/samba/smb.conf: check if the line security = user is set in the [GLOBAL] section
4. Set your shares in /etc/samba/smb.conf, see example

Example shares:

[allaccess]
    path = /media/common
    read only = no
    writeable = yes
    browseable = yes
    valid users = one, two, three, four
    create mask = 0644
    directory mask = 0755
    ; if you set this, all files get written as this user
    force user = one

This will be accessible via \\yourserver\allaccess

A single user share:

[special]
    path = /home/two/onlytwo
    read only = no
    writeable = yes
    browseable = yes
    valid users = one
    create mask = 0640
    directory mask = 0750

Restart the samba server after the changes with:

sudo service smbd restart

I think you should change valid users = ashley joe %S to only valid users = %S. In your example, ashley, joe and %S [the name of the share] is always valid, so ashley and joe can reach all homes, any other (if there are) users should only see their own in the example you gave.