I think the first question one should ask is what is the purpose of this book.

Is it to promote APUG?
Is it to promote analogue photography?
Is it to increase the number of subscribers?
Is it to make a profit for APUG to invest in the site?
Is it to constitute a nice recollection of members of a community?

Ends do matter.

If you want to promote analogue photography, you consider sections for alternate processes and weird stuff although the printing technology will certainly flatten differences.

If it is to constitute a recollection, you allow 1 or 2 pictures of a large number of photographers, and it could be a "how we were" recollection album;

If it is to sell the book itself as a profit for APUG, well, as above, because you will sell at the very least one copy for each participating photographer. More photographers, more sales. Purchase of at least one pre-paid copy could be a condition for participation.

If one wants to increase subscription numbers, again, most participants will yield most new subscriptions.

If you want to promote the site, or analogue photography, then you might aim "high" and go for "quality" whatever that means (and it must be a bet because "beautiful in art" is a very subjective thing). But that can have side-effects. Most photographers feel they are artists, and most artists think they are good artists and the worlds doesn't understand their greatness. A selective process risks to touch nerves and even alienate some (narcissist) people from the site

(my €0,02).

I don't plan to participate. I feel an honest artisan But I think the entire initiative is very interesting.