Group private chats should be added. Really it would be fun for role play and community chat without having a message lost in a sworm of random ones and inviting people to your messages with all consent should be an option too.
I know but talking from what would be more convenient it would be better for the invited person to see what happened before to continue the conversation smoothly without an unnecessary explanation.
@Lupine is correct. When you create a new conversation you can add as many people as you want. You cannot boot someone from a conversation but you can leave and not accept anymore messages from that person