How to store multiple foreign key values in a single field?
The best way to do this is with a one-to-many relationship message_recipients table.

user_info
( 
  user_id INTEGER PRIMARY KEY, 
  email VARCHAR(256), 
  password VARCHAR(256)
);

messages
( 
  message_id INTEGER PRIMARY KEY
  time_stamp DATETIME, 
  message VARCHAR(4000), 
  sender INTEGER -- references user_info.user_id
);

message_recipients
(
  message_id INTEGER, -- references messages.message_id
  recipient_id INTEGER -- references user_info.user_id
);