There is a quick yet counter-intuitive way to check whether a vertex with a certain index belongs to a group or not. Whenever you call
your_object.vertex_groups[GROUP_NAME].weight(i) to check the weight of a vertex, if that vertex is not present in the mentioned group, this method raises a
RuntimeError. You can use it. Here's the code:
for vert in verts:
#if we get past this line, then the vertex is present in the group
vert.co.x = 5 + offset # or whatever value you want
# vertex is not in the group
Basically, you iterate over all the vertices in your mesh, trying to get their weights (but here we don't save them to a variable, because for your particular task you don't need the values of weights). If a vertex is present in your group, the
weight() returns its weight value, which is discarded immediately, and then it moves to the next line where it translates the vertex along the x axis. But, if a vertex is not present in the group, it raises
RuntimeError exception, thus skipping the vertex translation part.
pass means "do nothing", so it simply moves along to the next vertex.