Can't wait to read some of the replies to this. These are just my opinions so don't take them as fact, there will be plenty of different viewpoints along shortly...
Will there be groups to ride with?
Yes and no. It's going to be much smaller than PBP (100 riders vs 5000) but small groups of similar paced riders will still form. You'll have to work at it though, be prepared to chat to the other riders and share the workload (or apologise for not being able to do your share). Silent wheel-sucking will win you no friends.
Also, with a much smaller field there's less chance of jumping between groups if, for example, some people want to stop at a control for longer than you want to, or vice versa.
If not (or, heaven forbid, I get dropped) , is the route well marked ?
No, Audaxes in the UK aren't marked at all. One or two controls may be marked but it's usually obvious from the number of bikes piled up outside. See next answer though...
I hope to use my newly bought Garmin 705 but have yet to master the GPS function.
Master your GPS and you'll be fine, there are plenty of people on here who will give you advice on how to use it (tracks/routes/etc), how to make it last for multiple days (you'll need an external battery pack of some sort as it is unlikely that you can rely on recharging it by plugging it in anywhere).
I use an Edge 705 for Audaxing and, properly setup, you shouldn't need the routesheet except as a backup.
Navigating by routesheet isn't that difficult but you'll definitely need to have some idea how to do it just in case.
What kind of weather to expect ?
The UK climate is changeable, Wales even more so. If you did PBP 2007 then that's one end of the scale of what could happen (I didn't ride it but I read the stories).
Plan and prepare for rain but bring suncream. It may be cool at the top of the passes (you only go up to 600m elevation max) and you'll get cold on the descents if wet. Temperature drops quite a bit after dark (see below) but never below freezing in July.
Here's a photo of Snowdonia (you'll be going through here on day 1):-
http://www.greenbank.org/misc/IMG_0910.JPGQuite a few people got a bit of sunburn that day. Later that night it rained (the faster riders would have missed that). On another day you'd get 30mm of rain there.
Daytime temperature should be mid 20s
oC but a rare heatwave could push that up to low
o30s C, at night it shouldn't go below 5
oC but that means you'll need an extra layer (arm/leg warmers, windproof gilet/jacket and possibly a skullcap/headwarmer).