There are a number of things that could cause this:
1. Is the mdb marked as read-only? Right click the mdb file in Windows
Explorer and ensure that read-only is unchecked.
2. Do you mean that when one person has the file open, no one else can open
it? If so, ensure that all users have full permissions on the folder where
the mdb is located. Check that the first person isn't opening it
exclusively (using File, Open, Open Exclusive or opening it via a desktop
shortcut that uses the /excl switch).
3. Do you mean that when one person has the file open, no one else can make
any design changes? This is a new feature, starting in 2000. You need
exclusive access to the mdb to make design changes. Usually you'd split a
database into frontend/backend. Give each user a copy of the frontend. You
can then open your copy of the frontend, and make design changes without
affecting other users. Once you have the changes done, you can copy the new
frontend to other users.