The TeX Users Group (TUG) is a not-for-profit organization for TeX users word wide. TUG was the first TeX users group, founded in 1980, and it is the most international one, being an important part of the worldwide TeX community.
TUG supports the TeX software archive network CTAN, produces the print journal TUGboat and the TeX Collection DVD which contains TeX Live, a TeX distribution where also TUG members work hard on. Its membership income is also partly used to directly support development projects.
Stack Exchange, a network of currently 63 question and answer sites on various topics with more than 1,180,000 registered users, hosts also a TeX site: tex.stackexchange.com aka TeX.SX. This site now has about 6,600 users. On the TeX.SX meta site, the idea has been brought up, to join the TeX Users Group with the whole Stack Exchange Network, as an institutional member.
This idea has been supported by many TeX.SX users, showing consent by voting and suggesting ideas. So SE was convinced and decided to join TUG. Together with Stack Exchange, eight TeX.SX users will individually join. On Meta we are currently discussing about how to determine these members.
Now Stack Exchange is a member of TUG, which can be seen in each following issue of TUGboat and on this list: Institutional members of the TeX Users Group. Thanks to Stack Exchange for supporting and sponsoring the TeX Community!
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Note that the TUG Development Funds have funded some important and valuable projects, such as some of the microtypography in Pdftex and creation of the Inconsolata font. They are well worth supporting.
If you join a local TUG, they are all affiliated with and financially support the main TUG.