Lewis & Short

astĭpŭlātor (adst-), ōris, m. [astipulor].

  1. I. One who joins another in a stipulation, Gai Inst. 3, 110; so id. ib. 3, 117.
    Hence,
  2. II. An assistant in a trial, in gen.: testes tot … cum adstipulatore tuo comparabuntur? Cic. Quint. 18, 58; so id. Pis. 9.
    And trop., one who assents to or agrees with: illud falsum esse et Stoici dicunt et eorum adstipulator Antiochus, Cic. Ac. 2, 21, 67: vanae opinionis, Val. Max. 7, 1 fin.