How does seating allocation work with Ryanair if travelling in a group of 6, checking in online and not opting for reserved seats?
It has indeed changed one year ago. From https://www.ryanair.com/en/terms-and-conditions/regulations-reservedseating/:
ALLOCATED SEATING - ALL FLIGHTS FROM 8 APRIL 2014 ONWARDS
Customers who do not wish to select and purchase their preferred allocated seat, can check-in online between 7 days and 2 hours before each booked flight and will be allocated a seat free of charge.
I have always booked alone, but I imagine that if you have booked your plane together (selecting more than one passenger in the booking form and paying together) you will be given close-by seats.
If you are multiple people flying together but you have booked separately, you are out of luck. If I were you I would try (for science) checking in online all at the same time, to see if the seats are allocated consecutively or if you can infer a pattern. Please report your findings to us then!
EDIT: more useful links:
Edit: newer answers indicate that Ryanair have changed procedures. Please note that this info may be out of date
Ryanair seating typically works in that at some point, people are invited to board, a queue forms (it may have already done so) and everyone piles on the plane, taking seats as they find them. If it is important to sit as a group, then your best bet is to pay extra for priority boarding. Otherwise, just sit where you can. Unless among the last to board, finding pairs of seats is not normally a problem.
Note this is all based on my experience of having frequently travelled with Ryanair for a few years, but most recently over a year ago. I bathmat from Ryanair PR and your comment about optional reserved seating that their procedures may have changed.