One very common myth among Buddhist converts in the west is that monks and nuns may only teach the Dhamma if invited. There is absolutely nothing in the monastic rules to support this idea. It most likely came as a PR move to make Buddhism seem less threatening. Unfortunately it is now used as a criticism of good monks and nuns who confidently share the Dhamma out of compassion as the Buddha intended.
Now, there are, of course, proper situations for teaching and improper ones. Many guidelines for this can be found in the Vinaya rules. Most have to do with respect for the Dhamma. None of them have to do with being invited.
And of course we have to be skillful and make sure that our intentions are themselves in line with the Dhamma.