For about 2 years I have been part of a local running club, and I would recommend it highly.  It is probably different with every group, but I really think it has brought my running to a much higher level.

To address your specific questions:

What are your pros / cons to group running vs running solo?

Accountability, for sure, is the big pro.  Also I find it much more enjoyable to do a long run with another person, it is a good distraction, taking your mind off counting off endless miles.  I do listen to podcasts/music by myself, but I find the time flies by with a partner or two.  Also, it's a good place to hear about different events, training plans/strategies, and just generally discuss running all you want-- plus, it's a good way to make new friends, or find buddies to travel to a race with if you want to do one in another city.  Cons... Well, sometimes you adjust your schedule to make sure you can run with the group, and then no one shows up who is at your pace (this may not apply to you, but I am one of the slower of my club's runners).

How do you handle long runs when the group run doesn't give you that LR mileage you need?

I am lucky in that my group meets at a park which is about 2 miles from my house.  I run there, run however many miles with the group, and then run back-- or, since I am in a city with good public transit, I'll take the subway back.  If I need a couple more miles I'll run a loop (or more, or less) around the park.

Do people do more talking than running?

Not sure exactly what you mean by this-- In my group the conversation is often pretty constant, with silent patches once in a while (depends on who you're running with, obviously) but we are running the whole time.  Most everyone in the group runs their long runs at a conversational pace-- if they want to pick it up for the last mile or two they'll let whoever they're running with know, so they have the option to go with them or not.

Do people still listen to music / podcasts or whatever or is the expectation that people will run without headphones?

I think I would probably think it was a little weird if someone showed up wearing headphones and listening to music/podcasts.  Some people (who use the GPS on their phone) will have a headphone in to hear their pace but still contribute to the conversation.  To me, if you're just going to listen to music, you might as well run alone.  I do bring an ipod with me in case I am the only one running at my pace or if I end up leaving the group a bit early (or end up doing a loop or two afterwards on my own).

Fuzzy nailed it.

As for adding mileage, just do the additional mileage before or after.  I prefer before since many times we will descend on a coffee place or restaurant en-mass for post run meals/drinks.  You don't feel so self conscious dirty and sweaty in a big group.

Group runs have become an anchor point in my social life.  Many good relationships stem from people I run with.

I used to run with a particular group of women every Saturday, but I started doing runs very early in the morning during the week before work, which involves me getting up WAY early (like 4 or even 3:30 a.m.).  That being said they usually meet at 7:30-8 a.m., and I just want to sleep in, start whenever I'm ready to, and not worry about having to meet anyone.  Plus, I'm cranky and unsociable.