rec.org.mensa is a newsgroup in Usenet that is supposedly for discussions regarding Mensa, though there has been little content in this category in recent years.
The group was not created or endorsed by Mensa, but like other newsgroups, was created by people wishing to discuss the topic related to its name. Some Mensans participated in it in its early years, but access to read or post to it was never restricted in any way, and non-Mensans have always dominated. While there were some intelligent, Mensa-related message threads in it, there was also a lot of Mensa-bashing, and also a great deal of off-topic material with no connection to Mensa which could sometimes get vulgar and childish. Ultimately, the rising tide of spam, trolling, and other nonsense proved too much for the Mensan regulars, who eventually quit and left the group in its present state where it's rare for Mensa even to be mentioned at all in the course of the political rants, conspiracy theories, and other spouting that is done by the people who still use this group. Due to the large number of Mensa-related forums, both official and unofficial, that can be found elsewhere on the net, there has been little interest on the part of Mensans to try to "take back" this one.
Mensa officials have on occasion expressed questions about whether this group was in violation of trademark law by using the name "Mensa" without permission, but if this were the case, many other Usenet groups would be similarly in violation; there are newsgroups to discuss Disney properties that include "disney" in their names, for instance. Given the loose, anarchic nature of Usenet, it would be unclear who to sue, anyway.