Nobody likes spam, and we are strongly against it - that's why Subscribe-O-Matic (as the name implies) is a strictly opt-in service. The only way users can become members of your Subscribe-O-Matic group is to subscribe by touching one of your kiosks in-world.
We don't currently support any way of manualy adding members since that can easily lead to unsolicited messaging and spamming which by the way is also forbidden by Second Life Terms of Service.
We do however, offer a one-time import of members from your existing in-world group or other similar services and products. If you are interested in migrating your existing users to your Subscribe-O-Matic group, please check this article for more information.