Since someone needs to be able to write to the drive, I am going to assume you are the power user. I am also going to assume that when you ran the Setup Wizard you created a name for the Workgroup and you user the name "Family".
If my notes are correct, when Setup finished, you probably logged in again as user admin. If you select the ADVANCED tab, one of the options on the left is USERS/GROUPS and here you can add each of the users with a password. (When I did this I created each user with the same name that was used on the Windows machine.) Next, select the Groups option and name the group "Family" followed by the Add option.
After the users and groups are created, you should be able to select GROUP SETTINGS option. Here you can select the Family group, then select "All Accounts" and the ADD option.
I think at this point the only thing that needs to be done is to select NETWORK ACCESS from the options on the left and review the permissions of each users. I forget the default setting, but this screen should allow you to make changes. Hope this helps.