Also keep in mind there are usually more than 1 grep on systems...

Do a 'man -k grep' and boggle

On mine, it came up with:
man -k grep
bzegrep [bzgrep] (1) - search possibly bzip2 compressed files for a regular expression
bzfgrep [bzgrep] (1) - search possibly bzip2 compressed files for a regular expression
bzgrep (1) - search possibly bzip2 compressed files for a regular expression
egrep [grep] (1) - print lines matching a pattern
fgrep [grep] (1) - print lines matching a pattern
grep (1) - print lines matching a pattern
grep (1p) - search a file for a pattern
grep (rpm) - The GNU versions of grep pattern matching utilities.
grepdiff (1) - show files modified by a diff containing a regex
grepjar (1) - search files in a jar file for a pattern
msggrep (1) - pattern matching on message catalog
pcregrep (1) - a grep with Perl-compatible regular expressions
pgrep (1) - look up or signal processes based on name and other attributes
pkill [pgrep] (1) - look up or signal processes based on name and other attributes
zgrep (1) - search possibly compressed files for a regular expression
zipgrep (1) - search files in a ZIP archive for lines matching a pattern