Answer:
Holding an open morning inviting people to drop in.
Have a plant sale that highlights what you’re growing in the garden.
Invite different groups in for a gardening session such as active retired, local schools, scout groups or nursery schools.
Offer training, accredited or otherwise (contact your local ETB (was VEC) to enquire about funding a tutor.
The more options you give people for hearing about your community or urban garden project, proposed or current, the better. Not everyone reads the local notes or parish newsletter so if you only advertise in these two publications, you’ll miss a lot of people.