Acts 8:29

εἶπεν δὲ τὸ πνεῦμα τῷ Φιλίππῳ, and the Spirit said unto Philip, i.e. by some inward prompting.

πρόσελθε καὶ κολλήθητι κ.τ.λ., go near and join thyself to this chariot. No doubt this royal treasurer had a numerous retinue, and a single traveller on a desert road would be doing what was natural in attaching himself to a train of people who were journeying in the same direction. Philip would therefore be able to approach and hear what was read without being deemed an intruder.