Go to Settings → Users & SSO. The Users tab lists everyone in your firm. Invite user is at the top right.
The three fields
Email is required. It is the address the invitation goes to and the address they will sign in with.
Display Name is optional and is how their name shows in the app. It is only used when inviting someone new.
Role decides what they can do. It defaults to Preparer, which is the right answer for most people. If you are not sure, read User roles, and what each one can do before you pick. You can change it afterwards.
Click Invite user and they are added straight away.
What happens next
They get an email with a link to set their own password. That link is how they get in, so nobody has to send anyone a temporary password.
Until they sign in for the first time, their row shows an amber Invited badge. It turns green and reads Active once they have signed in. If the invitation email does not arrive, they can use Forgot password on the sign-in page with the same address, which gets them in without a new invitation.
What it costs
Every person you invite takes a seat, whatever role you give them. A Read-Only user costs the same as a Technical Admin. The seat count sits at the top of the Users tab, reading something like 3 / 11 seats. See Your plan and your seats.
If the invite is refused
Two messages you might see, and what they mean.
"You cannot assign a role with capabilities you don't hold yourself." You picked a role that can do something your own role cannot. A Manager cannot create a Technical Admin, for instance. Pick a role at or below your own, or ask a Technical Admin to send the invitation.
A seat message. Your firm is at its seat limit. What you do next depends on how your firm is billed. See Your plan and your seats.
Setting up several people at once
Use the roster upload instead of inviting one at a time. It sets roles and client access for the whole firm from one spreadsheet. See Add your team in bulk.