Anyone know that app?

IDK about the app (never used it), But it has great reviews on google
https://g.co/kgs/s9swNx
The people who you want to contact should be on threema, So it faces the same problem as telegram. You cannot send message to everyone and have to invite them first :man_shrugging:

So cannot creating a backdoor to got in channel with two people when want to talking.