It's simple, but you have to also fetch all the keys and combine them. I suggest using async to make the calls asynchronous:

function getAllUsersInChannel_suboptimal(channelName, functionCallback) {

client.keys(channelName + :user:*, function(err, userKeys) {

    if (!err) {

        async.map(userKeys, 

            function(userKey, callback) {

                client.hgetall(userKey, function(err, user) {

                    if (!err) {

                        callback(null, user);

                    }

                    else {

                        callback(err);

                    }

                });

            }, 

            function(err, userList) {

                if (!err) {

                    functionCallback(null, userList);

                }

                else {

                    functionCallback(err);

                }

            });

    }

    else {

        functionCallback(err);

    }

});

}

Using a pattern match can drastically reduce performance even on medium sized databases. I suggest you also index the id's of the users using a set.

Create the counter key if it does not exist and set its value to 0:

SETNX <channel_name>:user:counter 0

When you add a new user, first increment the counter key, which also fetches its value:

INCR <channel_name>:user:counter => <new_id>

This will yield the new id for the user. Now you have to save a new member in the user:index set like so:

SADD <channel_name>:user:index <new_id>

You can now add the new user key like so:

HMSET <channel_name>:user:<new_id> <list of values>

To fetch data for all users you would first get a the list of existing users like so:

SMEMBERS <channel_name>:user:index => <list of all user ids>

For each id you would have to fetch the hash values:

asynchronous each <list of all user ids> as <user_id>

HGETALL channel_name>:user:<user_id> => all values of key

end asynchronous each

In Node.js, considering that you've already used the techniques above to index users, you retrieve all users in a channel using something like this:

function getAllUsersInChannel_optimal(channelName, functionCallback) {

client.smembers(channelName + :user:index, function(err, userIds) {

    if (!err) {

        async.map(userIds, 

            function(userId, callback) {

                client.hgetall(channelName + :user: + userId, function(err, user) {

                    if (!err) {

                        callback(null, user);

                    }

                    else {

                        callback(err);

                    }

                });

            }, 

            function(err, userList) {

                if (!err) {

                    functionCallback(null, userList);

                }

                else {

                    functionCallback(err);

                }

            });

    }

    else {

        functionCallback(err);

    }

});

}