You can convert any numeric integer primitive to its binary representation as a string with Convert.ToString. Doing this for each byte in your array and concatenating the results is very easy with LINQ:
var input = new byte[] { 254 }; // put as many bytes as you want in here
var result = string.Concat(input.Select(b => Convert.ToString(b, 2)));
Update:
The code above will not produce a result having exactly 8 characters per input byte, and this will make it impossible to know what the input was just by looking at the result. To get exactly 8 chars per byte, a small change is needed:
var result = string.Concat(input.Select(b => Convert.ToString(b, 2).PadLeft(8, '0')));
byte[]