I believe one of the best ways to circumvent such problem is to use online resources, such as social media (as mentioned by Luciana and Natalia), web groups and forums, mobile applications and so on. The cost can be close to nothing and, at least in the context from which I speak (São Paulo – Brazil), such channels have an enormous penetration in target populations such as MSM and TGW. It’s a cheap and effective way to ensure their participation.