A number of Landcare groups are now managing their memberships online. There are a number of open-source and low cost platforms which will enable you manage your membership list, allow members to make payments to your group and help grow your membership base.
Wild Apricot is web-based software for small associations and non-profits to help manage membership, website, events and other activities. It is "cloud" software, meaning it runs through a web browser without needing to install anything on your local computer. Pricing plans start at $40 per month.
The Mobile Landcare Group use Wild Apricot. You may like to contact them if you're interested in this.