I created this DB that is used by several people. Here are my questions..
How can i made it multiuser?
It's multiuser out of the box. You don't need to do anything.
How can i open with admin rights when others have it open?
Open it; if you have security implemented, open it with an account with admin
privileges.
What you can't do is have two people making *DESIGN CHANGES* to the structure
of the database simultaneously.
For a multiuser database you really should use the Database Splitter Wizard to
split the database into a shared backend containing the tables, and a frontend
containing the forms, reports, modules, etc. Each user gets their own copy of
the frontend; you would make design changes in your copy of the frontend
(opened exclusively). When you're satisfied with the design, you can either
give a copy of the new frontend mdb file to each user, or (better) create a
MDE file and distribute that.
See
http://www.granite.ab.ca/access/splitapp.htm for a thorough discussion.