In Edit Mode you can use Select > Select More/Less > More to add all vertices that are connected to the selected vertices to the selection. How can this function be replicated in geometry nodes?
This can be done with the Interpolate Domain node.
Each frame of the gif is created by connecting the nodes A, B, and C to the output. This works by using two Interpolate Domain nodes; the first converts the value to a float on the point domain, the second converts to the face domain, which uses the average of all connected vertices as it's value, then rounds that value to 1 because it is set to boolean. The result is expanding the selection to all vertices that share a face with an already selected vertex. You can also use the edge domain to expand to vertices that share an edge.
Selecting less works the same but you invert the value before and after the Interpolate Domain nodes.
Solution created in Blender 3.4. Should also work in 3.3, but the viewer node works differently so the results would have to be visualized another way.