It all comes down to how you design your application. There are a couple helpful things in the app store that help with building a multi tenant app.
Here is an administration module that supports multi tennants
also here is a nice widget that allows you to have multiple themes in your project ( allowing you to switch between themes based on which user a company is from
These are the only two that I know of, but here is a link to the app store. Maybe you will find some more widgets or modules that will be helpful
first question you have to ask is: What is exactly required in terms of multi-tenancy? The term is quite generic but often used for very specific requirements.
Generally you can implement multi tenancy with Mendix (See links by Austin) by correctly applying security models.
Two things to note: