If you are the account holder for your organisation, team members can be added to your company's account via your Teammates page in Settings.
You can also access this, and all other pages in your Account & Settings, by clicking on your company name at the top of the left-hand navigation.
Here you can view usage for your entire account (see more info on usage here).
You can also flexibly add and remove an unlimited number of users, providing their email domain matches that of the account holder (more info on unlimited users here).
To add a new user, simply click the purple 'Plus' icon and copy the link provided.
If you need to remove any users, find their name and click on 'x' icon in the 'Action' column. You will be prompted to confirm this before the action is completed.