Alex,
When you join a group as well as the hourly rate you pay for the aircraft (wet or dry) you will also have to pay a monthly standing order. The monthly standing order will vary in amount depending on the aircraft and location/type of storage but insurance is generally covered in this.
When you find a group ask them what the standing charge is and what it covers, havent heard of any groups myself though where you are expected to sort out your own insurance.
Julian.