With the database splitter wizard you will have to split your database into 2 files front and back end. Forms and reports should be on the front end. Tables, indexes etc will be on the back end. Tables in front end should be links to the back end tables. More here:
https://support.office.com/en-us/article/Split-an-Access-database-3015ad18-a3a1-4e9c-a7f3-51b1d73498ccThen you should use the tool from
www.autofeupdater.com so that each user works with their own copy of the front end.
You should not use the mdb file simultaneously..