Surprisingly, there's a quite a handful. Guido, Craban, Mercutio, Chayenne, Knightmare, Grimrath, and Solkrin were all Tibia players first before they were Cipsoft employees.
I mentioned present and former Cipsoft employees in my answer because they still count as they helped shape Tibia for what a great game it's worth today.
Please see my sources below for additional information-
Knightmare was one of the first Tibian players and is known for helping CipSoft over many years as a gamemaster, map developer, quest designer, and later, officially hired in CipSoft. During Tibia's 15th and 20th Anniversaries, an NPC named Knightmare appeared in-game.
I found an article where the graphics artists used to work with external artists, however they remain unnamed and I guess not officially apart of the Cipsoft team:
Moreover, Jan is also working with a few very gifted external artists, some of whom are former Tibia players, who have been contributing sprites to Tibia since its early days.
In 1999, Steve, Stephan, and Durin got to know Guido through a common real life friend. As it turned out, Guido already had a character in Tibia and played the game in his spare time. They had some fruitful conversations about Tibia and its future development. Guido, being a physicist with a strong analytic mind, immediately sensed great opportunities for the whole project. He was so intrigued by what he heard about it that he joined the god team and started out as a game content designer helping to expand the game world and to create monsters and items.
It grew to an enterprise with more than 50 employees, many of whom had been hired directly from the player community like Craban, Mercutio, Chayenne, Knightmare or Grimrath. Solkrin, also being a former community member himself, was the very first employee at CipSoft. No, he did not start out as a programmer or a designer. Solkrin was hired for doing customer support! He joined in 2003.