Had to look that up as I didn't know what TSP was,
https://www.tsp.gov/tsp-basics/how-tsp-fits/
Participant eligibility
Most employees of the United States government are eligible to participate in the TSP.
You are eligible if you’re any of the following:
- A FERS employee (generally if you were hired on or after January 1, 1984)
- A CSRS employee (generally if you were hired before January 1, 1984 and did not convert to FERS)
- A member of the uniformed services (active duty or Ready Reserve)
- A civilian in certain other categories of government service
In addition to being covered by an eligible retirement system, you must also be
- actively employed by the federal government as a civilian employee or as a member of the uniformed services,
- in a pay status in order to contribute, and
- working full- or part-time.