Dropdown and filter

I have the following problem:
I have a database shared with different users that have the option of editing and adding contacts.
I have many entries with dorpdown entries just because we need to filter the information.
My problem is that the information in those dropdown field may vary according to the needs. For example, add a new name of a company that is new for us.
The problem is that when the users want to add this new information and have to add it in a dropdown entry, they can not do it (it is only the option of chosing an entry). As a normal user is there any possibility to add new entries to a dropdown field?