Hello Dianna, In a multi-user environment it is always best to split the database. 20 MB is not a big database at all. Place the back end on the server and you can distribute individual front ends for each user. That will work for those connected to the LAN.
Sharing a live Access database externally is a whole different obstacle which Access was not meant to do.
The only solution I can think of would be setting up a remote desktop computer on the LAN and giving the external users a way to log in. Not simple or perfect and probably requires some IT support.
Not sure if that helps. We can set up a session to review if you would like to.
Jeff