You can use the chpasswd command. This command can only be used as root. Enter the chpasswd command into the command line then enter the name of the user followed by the password, do this for each user. When finished type ctrl+d. ex.
your-computer ~ # chpasswd
Another way that you can do this is to have a file containing all of the usernames and passwords, int he same format ie. user1: password. create this file using any text editor such as vim or cat.
Example on creating the file.
your-computer ~ # cat > bulkpasswords
Then run the chpasswd command(as root) on the file:
your-computer ~ # chpasswd < bulkpasswords
Hope this helps!