You can't set someone else's password. They reset it themselves from the sign-in screen, and it takes about a minute.
Nobody at the church can set another person's password, and nobody at Unison can see one. The box that did this was removed: one person setting another person's password is exactly how an account gets taken over, and the server now refuses the change even if something asks for it.
What to tell them. On the sign-in screen, tap Forgot your password?. Unison emails a link to the address on their account, and the link lets them choose a new password. They are signed in straight afterwards.
If the email doesn't arrive, ask them to look in spam first, then check the address on their tile under Team is the one they actually read. If it is wrong, correct it and ask them to try again.
Somebody who has never signed in has no password to reset — they have an invitation waiting. On the team list they show under Invited. Tap Send again to email it to them once more. An invitation is good for 7 days.
Changing a password you already know is a different thing and they can do it themselves: their name at the bottom of the sidebar, then Your profile → Account → Change password.