The short answer
Adding Users to the entire company domain (Account Level):
Open Settings using the gear icon at the bottom of the left sidebar, then click Users under Account and choose Add user. Enter their email, pick a role, and they will get an invitation to join.
NOTE: This will give the user access at the Account Level which means they will be able to see all projects.Adding Users to Specific Projects (Project Level):
Open Settings using the gear icon at the bottom of the left sidebar, then click All Projects under Projects. Select the project you would like to add a user to - this will reveal the project roster. Select Add user - Enter their email, pick a role, and they will get an invitation to join.
Who can do this
You need to be an Account Admin or Account Collaborator to add people to an account, or a Project Admin or Project Collaborator to add them to a single project. If you do not have one of those roles, ask a colleague who does, or start a chat from the message icon in the lower left.
More detail
See Add Users to an Account or a Project for the full walkthrough, and Account Roles and Project Roles Explained if you are not sure which role to give someone.