Erlang stdlib provides string:join/2 function but not binary:join/2. Below are few options how to do binary:join/2. Guess which one is the fastest.

binary2.erl

-module(binary2).

-export([joinl/2]).
-export([joinfl/2]).
-export([joinfr/2]).

% use list comprehension
joinl([], Sep) when is_binary(Sep) ->
    <<>>;
joinl([H|T], Sep) ->
    erlang:iolist_to_binary([H, [[Sep, X] || X <- T]]).

% use lists:foldl/3 followed by lists:reverse/1
joinfl([], Sep) when is_binary(Sep) ->
    <<>>;
joinfl([H|T], Sep) ->
    erlang:iolist_to_binary([H, lists:reverse(
        lists:foldl(fun(X, Acc) -> [X|[Sep|Acc]] end, [], T))]).

% use lists:foldr/3
joinfr([], Sep) when is_binary(Sep) ->
    <<>>;
joinfr([H|T], Sep) ->
    erlang:iolist_to_binary([H|
        lists:foldr(fun(X, Acc) -> [Sep|[X|Acc]] end, [], T)]).