(1)

ifstream KeyRead;
KeyRead.open(EncrKey);
if(!KeyRead.is_open())
{
cerr << "File failed to open for read. Program ending." << endl;
system("pause");
return 1;
};

A file can be open but still in a failed state. You should read up on failbit, eofbit, etc. A common way to check for a properly opened file would be more like

if(! KeyRead) because operator! () will return true if failbit or badbit are set.

(2)
char OutFile[15] = "Messagex.encr";
....
FileTestTries++;
OutFile[7] = FileTestTries+48;

This is goofy. Take the time to write a real (and genuinely useful) function that appends numbers properly to the end of a filename string. Using std::string this is brain dead simple and then just us string.c_str() if you need to dump to char* strings.

(3) int FileTestLoop - use bool with true/false.

(4) ofstream Write, KeyRead; -- *really* bad variable names. Outfile, ofile, something like that. You can easily be using a library some day that has some function named Write(). Not a show stopper but get in good habits now.

(5) for(int z=0; z<UserInCount; z++)

Minor but ++z is slightly faster so try to get into the habit of using it in loops like this.

It's late and I can't read any more but in general it looks like a good effort. Keep going and keep learning. You are off to a good start.