As John Henderson points out to us, this paragraph initiates a narrative stretch in which a rhythmic pattern of ‘ins-and-outs’ (or ‘es and ads’) bursts out all over through the to-and-fro of the storytelling, dancing attendance round Nero:  adigebatur - adeptus - eliceret - e proximis coloniis - acciverat -  - evenit - egresso - adfuerat - edebatur - adsumptus... edicto - adiit - aditurus - egressus - adversum - evenerat - acquireret - e terris - adstabant - …
Tacitus here takes a step back. Nero’s desire to appear on stage may have been driving him on, but even he has not entirely lost a sense of decorum. He does not dare to inaugurate his career as a public performer in Rome but chooses a Greek city famous for its Greek entertainment culture instead. Tacitus presents this choice both as an avoidance of Rome and as an anticipation of Nero’s trip to Greece, which would happen several years later (AD 66–67).
The sentence features a series of subjects: (i) vulgus, which governs the perfect participle contractum; (ii) the implied antecedent of quos, i.e. ei; (iii) the implied antecedent of qui, i.e. ei; (iv) manipuli. They all go with the main verb at the end: complent. The et links vulgus and the first implied ei; the -que after qui links the two implied ei; Tacitus then continues, climactically, with etiam (‘even’). The pronounced polysyndeton magnifies the list of those co-opted to swell the emperor’s enormous retinue. Tacitus revels in the idea of so many men from so many different groups flooding into the theatre of Neapolis.