ἐπισύστασιςepisýstasis

a conspiracy, i.e. concourse (riotous or friendly)

Greek (New Testament) · G1999

Meanings

a gathering, a riotous throng: Rec. (for ἐπίστασις, see that word), Acts 24:12, 2 Corinthians 11:28.

Same root

ἐπίepí · G1909properly, meaning superimposition (of time, place, order, etc.), as a relation of distribution (with the genitive case), i.e. over, upon, etc.; of rest (with the dative case) at, on, etc.; of direction (with the accusative case) towards, upon, etc.
συνιστάωsynistáō · G4921to set together, i.e. (by implication) to introduce (favorably), or (figuratively) to exhibit; intransitively, to stand near, or (figuratively) to constitute

Source: G. Abbott-Smith, A Manual Greek Lexicon of the New Testament (1922), public domain.