Nick, depends what level you want to marshall at?
For rec-ball it's probably just worth contacting your local site and offering your services. Most sites just need reliable people that can turn up as often as needed and are good with the punters. Because a good marshall can be the reason that a punter does or doesn't come back to the site.
Back in the day, i've played at sites where the marshalls were twats and they got no further business from me, compared to other sites where the marshalls look after the punters and have a laugh......after all that's what it's all about.
You still interested in playing tourney, nick?