calling a C program in a shell script

I'm assuming that the program is actually compiled. If it is actually compiled, it is just like calling any other program...

As J.Ja points out, you just call it by name. In fact, much of what you do in a shell script actually is a C program. If you're talking about running a binary executable file produced from C source code from a shell script, you're just talking about running a "command" from that shell script. Things like ls, cd, and mv are all programs written in C.