userChatId

Identifier of a private chat with the user who sent the join request. This number may have more than 32 significant bits and some programming languages may have difficulty/silent defects in interpreting it. But it has at most 52 significant bits, so a 64-bit integer or double-precision float type are safe for storing this identifier. The bot can use this identifier for 5 minutes to send messages until the join request is processed, assuming no other administrator contacted the user.

type: Integer