Hi Paul, Zwiftpower was a third party results service which race / ride organisers use to set up their races, it contains a tool set so we can set the race/ ride parameters.
Zwiftpower became a part of Zwift last year. It has a list of all the ride / races which are on the platform so you can use it to find a race / ride which suits you.
It also shows rider details of speed, power, race finishing positions, riders who have been DQ so you can see how riders performed.
The easiest way is to search for it in google and then sign up, personally I think the sign up process is quite straight forward but others may disagree.
On the subject of Group rides, there will be some that will follow the ride parameters and some that don’t, the important thing is to always be close to the ride leader group, designated by a yellow arrow. If you do fall behind because of group dynamics it is hard to catch up again. Most rides will have a sweep team to help you. I would recommend Pack, ZZRC, ZZHC, Viking Valhallla, Revo but you will find ones which suit you. Remember, it’s all about having fun. Hope this helps.