If you sponsor somebody that is already participating, it will just grow their subscription (upvotes more frequently, or larger upvotes each time).
The easiest way to check a membership is in our discord server we have a tool that allows you to check any steem name for their SBI level.
If they are not already a member, we send a message letting them know you sponsored them, but we are not doing that for existing members yet.
Great, thanks for explaining!