Welcome to the forum!
This has been debated a few hundered times here but I will list the low down since this is your first post.
The shared db must be split. Each user must have a copy of the front end installed on their hard drive. Each front end must be linked to the same back end on the server. The back end has all the data tables, the front end has everything else.
Each user should also have a copy of the workgroup file *.mdw on their hard drive as well.
Each user must have full permission [add/edit/delete] to the directory that the front end and the back end is located.
Do not join the users computers to your workgroup file. Each user should use a custom shortcut to open the secured db with the correct workgroup file. Here is an example of a shortcuts target field [all on one line]...
"C:\Program Files\Microsoft Office\OFFICE11\MSACCESS.EXE" /runtime /wrkgrp "C:\Database\Workgroup.mdw" "X:\Database\Database.mdb" /user UserName
Searching this forum will provide you with a ton of threads to read about 'security' and the other posters problems and hopefully thier solutions.
Good luck!