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Thanks for reaching out!
Indeed groff can be installed using opkg from the official repo. In order to install this package, make sure you are on the latest image (201606081705) and that your repo is up to date (use the command opkg update).
If are up to date with everything you can install groff with the command:
opkg install groff
If you are having issues after this you can try installing the packages groff-dbg and groff-dev to see if this helps. If you still have issues after that, then try installing groff from source, you'll find more information about this at https://www.gnu.org/software/groff/
I hope this information helps you,