Were looking steadfastly (atenizontes ēsan). Periphrastic imperfect active of atenizō, a late intensive verb (intensive a and teinō, to stretch). Common in Acts and also in Luk 4:20; Luk 22:56 as well as Act 10:4, which see.
As he went (poreuomenou autou). Genitive absolute of present middle participle. They saw him slipping away from their eyes as the cloud bore him away.
Stood by them (pareistēkeisan autois). Past perfect active indicative of paristēmi and intransitive (note i in B instead of ei for augment, mere itacism).