Remember that entering a group and getting welcomed is a delicate matter, especially when you are promoting a product. Do not spam the thread discussions or the mails, but join in the forums and use a personal approach to get to your future customer.
Introduce yourself in the forum and give clear information about who you are and what the business you are in. Ask their inputs for your business to show that you appreciate the inputs from your customer. Give them special offer and services.
In the real world, you can contact the clubs near you. Use voluntary services as the means of promoting your business. A free lecture on how to take care of a rabbit will let the members of the Rabbit Society know that there is a new rabbit vet in town.
Keep providing services to the clubs so that they will remember you. This is a kind of promotion that costs nothing but time and effort. Be diligent and get as many exposures as you can to the society.
Using the internet and clubs as a start in promoting your business is the best and most effective way to be known. When you are already establish close relationship with your customer, they will go right to you when they need your product/ service.