So if they don't respond to your "do you have time to talk" initial message, then you just don't invite them to the concert at all and never tell them about it?
Yep, that's right.
All to avoid just asking them up-front whether they want to go or not?
This is where I have to correct you. Not asking them straight out, and not mentioning it later if they don't respond, are both is service of avoiding putting pressure on them.