User accounts can be members/users in multiple organizations at the same time.

On the web app, you can change the active organization by clicking on the organization selector button in the upper right corner of the page. The organization selector will list the name of the organization that you are currently accessing and allow you to select another one.

On the mobile apps, you can select what organization you want to access when you first sign in to your account.

Once logged in to your user account, you can change the organization using the 'Organization' buttons at the top of the Settings tab.