D&D encounters is intended to be run by a hobby store or something similar. This means, you can sign up and ask to be an Encounters DM. (I need to remember to do this myself). If you can't join the official encounters session, try turning a module into an encouterseque experience like suggested here.
When you become an encounters DM, you are asked to report back the results of your session back to Wizards of the coast.
You also get the following:
- The module for the week.
- 6 pre-made character 'sheets' Like
- A few progress sheets to keep track
of Encounter rewards that you might get after a few sessions.
My wife and I were able to learn the rules of the game while playing. Though it is helpful for atleast 1 player at the table to know the rules as well, it is not necessary. All relevant rules are given in the modules by Wizards of the Coast.
In general, encounters are broken up into 5 'encounters'. After the 5 encounters, all charachters level up. The encounters themselves are meant to demonstrate various aspects of the D8D game, and highlight new content. So while most of it is combat focused, it does introduce skill challenges, and dialogue and skill usage when it can.
I played it in my local hobby store, and we started of with 5 people. Eventually after about 2 months, we had over 12 people playing, with 2 dms and 2 groups playing side by side. The DMs would sometimes switch off so they could play as well. Also, we eventually allowed home built charachters for people who wanted those.
If you can apply to be an encounters DM I would highly recommend doing so. It's great fun. However, the 'story' only lasts for those 5 encounters. It then switches to a new story line, and often setting. (Though if I remember correctly, the dark sun campaign allowed us to level up twice)
I see you put up a bounty, which I assume means that you didn't find my answer complete, so I'm just going to give a summary of answers based on your exact questions. The details of which can be found in the above paragraph.
Question: Do the players need to know the rules and create their own characters?
Question: Is there a fixed group of players or does it change from session to session? Likewise, does one group always have the same DM?
Answer: By it's nature, the group of players and DM are likely to change from session to session, but they don't have to.
Question: Is there a continous storyline?
Answer: There is a continuous story for every 5 weeks. After those 5 weeks, the story can/might change
Question: If so, how is character advancement handled?
Answer: Advancement is handled at the end of those 5 weeks.
Question: And what happens to the PCs and the story between encounters?
Answer: You decide :)
Question: What if a player misses out on one or more sessions?
Answer: Encounters has a system of points which you gain for specific events during play. If you have X amount of points at the end of a 5 week 'campaign' then you get a reward that you can use on future campaigns. (Like curious knack which gives a one time bonus to a skill check) IF you miss sessions, you don't get the points, and have to wait longer to get the rewards. In other words, the Player is rewarded for playing, but the Character advances the same no matter what.
Question: Is it purely combat-focused or are there opportunities for dialogue and other non-combat activities?
Answer: It is mainly combat, with once in a while there being opportunities for dialogue and other non-combat activities.