We know that Russia was the second country to be involved in the Stargate program, long before any other nation had been made aware of it.
We know that the Atlantis expedition was supervised by the IOA, hence the members have been selected from several different countries.
Although there were some unlucky events with Russian nationals involved in the Stargate program, I believe that they would still like to have someone in the Atlantis expedition, especially when they know that the expedition and its discoveries may be unavailable for the Stargate Command (and therefore Russia) for some time due to problems with communications, even if they preferred to sit in the back, read reports, and watch.
Were there any Russian nationals in the Atlantis expedition? Is lack of them ever referred to in the series? Are there any in-/out-of-universe reasons why there weren't any?