It really depends on the hostel. Some places will not have any cooking facilities, some will have just a couple of stovetop burners and others will have pots/pans/stove/microwave/etc. Seems like HI hostels have less cooking facilities than many independents. You could e-mail the places if you really want to know. From what I’ve found, most people cook alone or in pairs, but many are willing to do a group meal if you ask around. Some hostels (rare) are even set up to regularly do group meals.

If you ask around before dinner time theres a good chance of finding takers on a group dinner it’s a nice idea but it’s not very common in my experiences.

Most hostels with cooking facilities have all the basics but kichens often get busy/full at dinner time making it impossible to do anything fancy or even beyond microwaving unless you like waiting.