Hi Skihorse,
In the UK, in order to take part in 'affliated' eventing, you need to register yourself (costs about £120 p/annum) and your horse (costs about £80 p/annum p/horse) with British Eventing. On top of this, entries are around £50 a go for the lower level (gets more expensive as you go up the grades) and you will need to pay a £10 start feel on the day as well so it is NOT cheap.
Classes in the UK used to start at Pre-Novice which was around Prelim dressage standard, 90cm XC and 1m SJ. Before this most riders would event at either unaffiliated or riding club level. However, the BE has recently restructured to scrap prenovice and include some smaller classes. BE80 has XC fences at 80cm and SJ at 85 cm. BE90 has XC fences at 90cm and Sj at 95cm. BE100 has XC fences at 1m and SJ fences at 1.05m.
After this you would move up to Novice level, the intermediate... then it gets seriously scarey
You don't need to have points in any other discipline but if you had them, it wouldn't affect you. You gain BE points whenever you win or get a decent place and once you have a certain number of points, you can no longer compete at that level and have to move up a grade.
Usually affiliated events are over subscribed so balloting is used to reduce numbers. You get a certain number of priority numbers with your membership and if you use this you are unlikely to be balloted. However, otherwise it can be a bit of a lottery.
Because it is so expensive, you can choose instead of becoming a full member, to to buy 'day tickets' which basically mean you can choose to compete at a certain event. However, you will be more likely to get balloted if you are riding on a ticket and you can only buy 4 tickets a season. On top of this, if you get placed, your horse will not gain any points.
I don't know if it is similar where you are but I know over here, eventing is the most expensive affiliated equestrian sport going which is why most people riding at lower levels compete unaffiliated. This is what I do... I have no interest in hurling myself over 1.05 solid obstacles anymore so my local riding club does the job for me perfectly.