Might be worth checking what's already available in your area - there might be easier ways of publicising, also finding out whether people turn out for any other social groups past or present.
If you go to lower right of Community page on NAS website there's a little red map that supposedly allows you to find groups and services in your area.
It doesn't work very well - I tried putting in social and Hull and got 250 things starting in Birmingham and Bristol before eventually finding Kingston upon Hull (NAS tells us the website is quality assured and reliable, but they've a funny notion of quality). I think the fact that it comes up with a lot of daft stuff first puts people off using it.
The Kingston upon Hull entry is for a meeting provided by Hull and East Yorkshire MIND (www.mindhey.co.uk). There may be others further down the list.
The MIND contact may be able to tell you the practicalities and options for setting up a group.