First, some history:
The Original series of "Whose Line Is It Anyway" was a 6-episode Radio Series on BBC Radio 4 in early 1988. It was hosted by Clive Anderson, with John Sessions and Stephen Fry as the 2 regular performers, joined by a pair of guest performers: Dawn French, Lenny Henry, Hugh Laurie, Enn Reitel, Nonny Williams, Jimmy Mulville, Kate Robbins, Griff Rhys Jones, John Bird, John Glover and Rory Bremner. Jimmy Mulville was the only guest performer to appear twice.
In September 1988, it moved to TV on Channel 4. Stephen Fry opted not to return, leaving John Sessions as the only regular performer. During the second season, John Sessions was phased out, in favour of a full roster of guest performers. By the eighth series, Ryan Stiles and Colin Mochrie had taken on the role of regular performers. The tenth series (filmed entirely in Hollywood, and consisting almost entirely of American comedians) failed to display much new talent, received low viewing figures, and the series was scrapped. Wayne Brady appeared in 5 of the of series ten.
(History lesson over)
Ryan Stiles then introduced the series to Drew Carey, who then successfully pitched an American version of the show to ABC. Drew Carey hosted the show, with Colin Mochrie and Ryan Styles as regular performers (appearing in every episode). Wayne Brady appeared as a recurring guest in 16 of 20 episodes in the first season. For the second through seventh series, he was a third regular performer (in every episode). For the eighth series, he was a recurring guest once more (17 of 21 episodes), and for the ninth through 15th series he has been a regular performer again.
(Also, while Colin Mochrie has appeared in every episode of the American series, Ryan Styles did not appear in 2 episodes of series ten, being replaced by Greg Proops)