Adding people
Team → Add member. You choose how the new member gets in, and the two options are not interchangeable.
Send an invitation — the default
They receive an email and set their own password. This is what you want almost always. If the email fails to send, you are told that it failed, not that an invitation was sent.
Set a password yourself
The account is created with a password you choose and no email is sent to anyone. That is deliberate: it is for handing someone their login in person. It also means that if you do not tell them the password, they have no way in and no way to discover one.
If somebody says they never got an email, check which way they were added. Somebody added with a password never had an email to receive. Send them a password reset from the Team page, or check their spam folder for the invitation.
Choosing a role
Pick a role when you add them; change it later from the Team page. The four built-in roles, and how to make your own, are on Roles and permissions.
Which forms they can use
A new collector can use every form that is open to the whole organisation. For a form with narrower access, grant it on the form’s settings — see View and Collect.
Removing somebody
Team → the member → Deactivate. They can no longer sign in. Their records stay, still attributed to them. Reactivate to restore access.