Navigate to the "Team" area by clicking on your profile picture at the top right and selecting "Team"

Click "Add Team Member". If you do not see this you may not be the owner of the team. Only team owners can invite new team members.

Select which accounts they should be added to

Click "Finish and Send Invite"

An email will be sent to them letting them know they have a pending invite.

When they have accepted, you will receive an email letting you know the invite has been accepted