if ur using phone you can see in top right corner there will be "3 dots" click on that and then a option will come for "joining this community" click on that then here whom ever you want to chat with mention their "id" like urs is @frankPal4101 and create a new post there who ever u want to chat with give the 'title' like
"Chat between me and @____" the person you want to chat with basically ❤️