string str = "1000111"; //this is your string in bits
byte[] bytes = new byte[str.Length / 7];
int j = 0;
while (str.Length > 0)
{
var result = Convert.ToByte(str.Substring(0, 7), 2);
bytes[j++] = result;
if (str.Length >= 7)
str = str.Substring(7);
}
var resultString = Encoding.UTF8.GetString(bytes);