First and foremost, I want folks to be able to see the member list without logging in. I've been to sites that do this with Roster, but for the life of me, I can't find the option in the control panel, nor the documentation that says its possible.

The "would love to have" feature would be for folks to login in phpBB and then be logged into Roster. I think this is less likely, but I thought I would ask.

There is no option for this in Roster
Roster is made as a standalone application and it will stay that way

However, that doesn't mean we don't try to help others in tying another app's login system into Roster

Roster's auth can be replaced with custom code via an addon
There is a 3rd party addon called "phpBB Login" in the 3rd Party Addons and Mods forum here
I do not know if the log on session is saved between Roster and phpbb, but they would be the same user/password

A search for "phpbb login" or "phpbb auth" would have helped greatly