Try cross-posting on your neighborhood board of Nextdoor, which is a location based social media platform. Also, put up actual paper notices at local bike shops and coffee shops, the library. Wherever there are bulletin boards.
Have a central meeting place, and a regular time. We are doing a group ride tonight at 5 PM. It meets the first Friday every month at a park in the middle of downtown. It is also full of activists and advocates. It is open to all riders. About 1/3rd and growing are on ebikes. Each ride has a different leader who has planned it. We take it easy and many go to beer gardens afterward. One ride's theme was hardware stores and grocery stores. The idea was that you can get around doing errands easier and faster on a bike than a car.