Team or Organization members can be invited by administrators via the settings page
It is worth noting early that starter plans are locked into only 1 user (administrator) and can not invite team members.
Navigate to the settings page by either clicking the left side panel or your avatar in the top left and clicking "Settings".
You should land right were you need to be!
Click the "Add Member" Button and enter a name and email and invite that user. - This user will now be invited via email and will have to follow those instructions.
You can control this member's portal permissions here as well!