You can grab a list of all of your clients who have been opted in to receive text message reminders by checking out your complete client list and exporting it to Excel! Once the list is exported, you will notice a column labeled SMS enabled. If the client is opted in there will be an "x" in the column. If they aren't, the column will be blank.
If you want to opt a client in to receive texts, you can do so right under their profile.
OR they can do that themselves in their scheduling profile.
If you are curious about just one client and don't need to pull your whole client list, then you can check their profile to see if they are opted in. If they haven't opted in, you'll see the option to send them an individual invite right there or you can opt them in yourself.