In the future, please feel free to post security questions in the Security
forum.
If you are using an .mdb, then you can use full Access User Level Security.
It is a pain, and it's not 100% secure, so it will only keep the honest
people out. I prefer to implement security within the application. Create a
login page, store their password in a table, compare it, and then you could
store the value in a variable. After log-in, you could store info into a
table about who logged in when, what they accessed, etc. (If you use AULS,
then you can use CurrentUser.)
Whichever method you choose, when the form opens, check that the user has
the proper permissions.