Thunderbolt is a combination of PCI express and DisplayPort.
To use eyefinity you need at least two screens to join. None of these needs to be displayport, but you obviouslu do need more than one screen else there is nothing to join with.
Now to the active/passive part and the actual answer.
On most PC's eyefinity is used with 3 screens (mostly used for games and two screens mean that many popups, cross-hairs etc in right in the middle of the two screens. Which is annoying.
Most PC cards come with two DVI and a DP port, or DVI, HDMI (which is a kind of DVI plus sound) and DP. To use a third monitor you would need the DP port.
Now here comes the catch: DP adaptors come in two flavours. One which actually does things and is more expensive and one which is simpler, but can not do a few things. The last is a passive adaptor and eyefinity does not work with it.
However you already start with Displayport (in the Thunderbolt connector). There is no need to convert anything. Simply connect a cable from the Thunderbolt port to the DP port on a monitor and you are fine.
Now if you have an older monitor without DP in then you would need to actively convert the DP port to a DVI or HDMI connection. A passive 'cable' will not do for this.