It used to be that when you sent a group invite, spelled everything correctly, and the player that was invited was online, you would get an invite sent message.
I understand that the other player online part was considered to leak info about others and so the msg was removed.

My suggestion is that the message appear no matter if the other player is online or not.

The only time the message would not appear is if some part of the invite command i.e. the player's name is spelled wrong

Yeah it makes sense to have some feedback wether or not your invite was sent. It should be fine if it's always displayed no matter if the player is online or not.