How to use "Screen command" for multiuser mode ?

Issue

• Need to know how to share one session with multiusers :

[user1@test ~]$ screen -r shared

 Enable multi-user mode
[user1@test tmp]$ Ctrl-a :multiuser on

 Add the desired user(s)
[user1@test tmp]$ Ctrl-a :acladd user2

 Tell the other user to try connecting to the same screen using the following command:
[user2@test ~]$ screen -x user1/shared

• How do I create the Ctrl-a command under screen session?