In the 1992 concert "Sondheim: a Celebration at Carnegie Hall",
celebrating the work of Steven Sondheim, the musical number "Good
Thing Going" is performed by a musical group called "The Tonics". I
wanted to get more information about the group such as if they've made
any other recordings, if they're still together, any links to info
about them.

Hello.

The Tonics were a Los Angeles-based vocal quartet featuring Brian Lane
Green.

From Mr. Green's web page:
"A vocal quartet was formed, "The Tonics", which appeared in various
clubs in the area!" Source: brianlanegreen.com:
http://www.brianlanegreen.com/Los%20Angeles.htm

The group is mentioned in several biographical sketches of Green,
including the following:

From showgate.com:
http://www.showgate.com/thelife/green.html

From culturevulture.net:
http://www.culturevulture.net/Books/BrianLaneGreen.htm

From cabaret.org:
http://www.cabaret.org/content/viewartist.phtml?artistname=Brian%20Lane%20Green

It appears that the Sondheim show was a career highlight for the
group. No albums are mentioned on the previous pages. Mr. Green has
done a solo album, though. And his recording credits are listed on his
web page.
http://www.brianlanegreen.com/Recordings%202.htm

Since Green's page doesn't mention any Tonics albums, it's a pretty
safe bet that there aren't any.

However, The Tonics did record song "Any Place I Hang My Hat Is Home"
for the CD, Over the Rainbow: Music of Harold Arlon.
http://music.barnesandnoble.com/search/product.asp?ean=21471291520

It doesn't sound as if The Tonics have been very active lately, as
Green seems to be primarily engaged in an acting and solo career. See
this interview hosted by Theatermania.com:
http://www.theatermania.com/news/feature/index.cfm?story=1730&cid=1

search strategy: sondheim, tonics, quartet, brian lane green

I hope this helps.