They'll throw out new sprouts from both runners and in some cases from seeds. You don't say what variety of Miscanthus sinensis they are, but if they're the non-variegated kind (just green) they should be showing a faint white stripe down the center of each blade.

Various forms of M. sinensis will set seed and in some cases, profusely. It is considered a potentially invasive species in many warmer parts of the country, especially in the southeast. It is supposed that much of the seeding is a reversion to species and is recommended that these seedlings be removed. If visible surrounding your plants, it is also possible that the seeds have spread further and have germinated in less controlled environments as well, hence the concern regarding invasiveness.

Your best and safest method of propagation would be by division.