If you're Account Owner or Administrator, click on "Settings" then "Invitations".
You'll then have to click on "invite member(s)" and fill out the form with your teammates email address, first name, last name, roles and team(s) if needed to.
☝️ No worries, once logged in the new user is able to modify his/her own information in his/her profil settings (avatar > my profile).
🤓 You'd like to invite your accountant? Follow the same procedure and give him/her a role of Controller. If you'd like him/her to be able to make payments then add the Requester role if needed to.