+1. A WhatsApp group with 10-20 ppl (in similar stages of life) worked well for a while for organizing hiking/tennis/squash/some sport/DotA. We got consistently 4-5 people including their spouses showing up. Usually 2-4 weekends a month. With that size, many people can comfortably pass on the invite.

Then organically these tend to turn into trips together or simple hangouts for someone's birthday or a holiday.