J,
Once a member has joined and hasn't created a password right at the
beginning, what's the easiest way for them to request one later on?
I would recommend that they go to any Groups.io page (its home page, or your group's home page) and click the Log In button/link at the top right.
/On the "Please Log In" page click the "Forget your password?" link. Then provide their email address and click the "Email Link To Log In" button.
They should watch their email (including Junk/Spam folders) for a log in email from Groups.io. Clicking through the link in that message will bring them to their Profile page where they can set a password.
That link will also log them in to Groups.io for 30 days (or until they log out); so they may put off setting a password if they don't want to choose one now. They can repeat the above process anytime, so there's no hassle if they accidently log out or let it expire without having set a password.
Or,
They can forget about setting a password, and use the "Email me a link to log in" button whenever they need to Log In.
Shal