I have been a member at Bedfont for a few years and you do sign up at the bar which tends to be open in the evenings.
If my memory serves me correctly, it was £20 for the first year and £10 a year thereafter and it is only a very short walk from Myrtle Ave..
The area you can use is not large and is pretty restricted if there is a football match or training on. As a member you can then use the bar and toilet facilities too. Whilst I have never had any issues,(I do not go that often) I have heard that they are less happy with you being about and taking photos when junior matches are on.