Are they sharing a Windows log in? If not then IMO, you can dispense with the whole idea - but it will take some admin maintenance. What I'm talking about is Access code (which cannot be seen in an accde type database) that gets the user's Windows login ID (user name) from a table and compares that to who is currently logged in to Windows and using Access on that pc. Note, I'm not saying you store windows passwords in the table, just their user name. If they don't match, they don't get in. This avoids admin maintenance when they call up and say "I forgot my password - can you reset it?" and a whole lot of code.
If stuck with the login form thing, then you need to do at least the minimal to keep users out of tables and playing with design, but that's only as good as the level of their knowledge about Access and vba. Then you need to encrypt passwords, but even I with my limited knowledge (at the time) was able to beat that. If security is of paramount importance, you probably should dispense with Access as a back end and use something else, like sql server.
Welcome to the forum, by the way!
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.