How much coding experience do you have? You can do this by creating a username and password table, or using some outside source for the username and password. Then giving a popup modal form when the app opens, and have the user login. Take the credentials, check them with your source, then open a hidden form and refer to that hidden form for rights. On your username/password source, you can also set rights at various levels (like "Administrator" or "User" group), and then various other discrete rights, and put those in the hidden form. Then, for example, when a form opens, have it check the access rights of the user, and then make certain things invisible if they don't have rights. A form's On Current event might have code like:
Dim sGroup As String
sGroup = [Forms]![frmHidden].[GroupName]
If sGroup = "Administrator" Then
Me.Button.Visible = True
Me.Button.Visible = False
'Set the button's Visible property to No to make sure.
The problem is, most of this is easily thwarted. The built in security is good for most things. The best security, if that is the issue, is to use SQL on the back end, and AD credentials to secure access to various objects and tasks.
Does this help?