To add user to the organization login to the Office 365 Admin center - https://portal.office.com/adminportal/home - and login with your credentials.
Go to Users -> Active users
Click Add a user
Fill basic information about the user and choose users' domain, fill contact information like phone, etc.
NOTE! The location must be picked in order to assign licenses. You can choose other locations for different users - it is recommended for enhanced routing performance (user data will be kept with all other users in your tenant GEO location).
Choose licenses to assign to a user:
If you chose auto-generated password it will be provided in the next step.
You can decide whether to send credentials via email or provide them to the user in some other manner. After sending or clicking Close user is created.
For all services to be configured it might take from 30 minutes up to 1 hour. User should be able to use most of them after few minutes but all options might not be available.