BeOS does not come bundled with the Unix-style man utility, though it does come with a small collection of man pages in HTML format (look in /boot/beos/documentation/Shell Tools/). If you’d like to get your hands on the real man utility, download Sebastien Bouchex’s port of man.
To get man running properly, you’ll need to have a minimal GeekGadets installation set up. The only GG component you absolutely must have is groff. Decompress groff in the GeekGadgets directory — its binary should end up in GeekGadgets/bin.
Once that’s taken care of, run the man install script and everything should „just work.“
The only thing you’re missing now is a collection of man files and a place to put them. man files are often included with command-line Unix/Linux utilities, and are included in almost all of the GeekGadgets packages. Drop man files into subdirectories of /boot/apps/GeekGadgets/man and everything will work just like man on Unix systems.

Note that the GeekGadgets collection also includes its own manutils port. However, that’s slightly trickier to set up than Bouchex’s port. The old instructions for installing GG’s manutils are preserved below.
Download the manutils binary from ftp.ninemoons.com/pub/GeekGadgets. Install man per the instructions in the tip Installing GeekGadgets.
In order for man to work, it requires that nroff is installed. Since nroff is part of the groff toolset, install groff as well. Unless you have the entire GeekGadgets collection installed, you’ll find that man looks for its own version of zcat, ignoring the one on your system. To get around this without installing all of GG, make a symlink from /bin/gzip to /boot/apps/GeekGadgets/bin/zcat.
Finally, you’ll need some man pages to view. At this writing, you’ll probably find it easiest to download man pages from the Amiga archive, also on GeekGadgets. Many shell utilities also come with their own man pages, and you’ll need to make sure man can find them. To modify man’s path, take a look in /boot/apps/GeekGadgets/etc/man.conf and edit its paths to suit. You should now have fully functional man capabilities on your BeOS system.
Also, according to Tom Spindler:
If you don’t want to bother with man itself but still need to view the occassional man file, just install groff from GeekGadgets. You’ll then be able to type:

groff -man -Tascii foo.man | less

Alternatively, you can use groff -mandoc. This provides a slightly different output format which some people prefer. If you omit -Tascii from the command above, output will be dumped to postscript format.
If you’d like to view man pages in HTML format, download and install Kristan Slack’s excellent NetMan utility after following the instructions above. You’ll then be able to launch man docs in NetPositive by typing simply netman commandname.