I searched the NumbersUSA site, and the only mention of the Sierra Club was a statement by a David Brower in 1966 that he believed that overpopulation harmed the environment. Why would you think that NumbersUSA is connected to the Sierra Club? If they were, so what? Their message about illegal immigration is a sound one.
Roy Beck and Leon Kolankiewicz co-authored a book together and have another web site at www.sprawlcity.org. You can check out the list of advisors at each site and find a number of sierra club members but Leon is a real piece of work, if you care to investigate his agenda.
Like Roy says in his bio, you have to know who your talking to.