I know guys, that's why it's already in it
!
Check lines 59 - 64 :
PHP Code:
if(MessageType == AS_NOT_FRIENDS)
{
int client = FindClientBySteamID(message)
CPrintToChat(client, "%s {green}I can't invite you in my steamgroup, because we are not friend. Please accept my friend request and try again.", MODULE_NAME);
ASteambot_SendMesssage(AS_FRIEND_INVITE, message);
}