The RHS website has a ‘find a local group’ search tool for local gardening groups which can be contacted via the website. https://www.rhs.org.uk/get-involved/find-a-group
The community gardens local to ecobirmingham, in Northfield, are Allens Cross Community Association and Masefield Gardens. These community spaces can always use an extra pair of hands, and offer a warm welcome. If these gardens are not local, try using the link to find a closer garden.