This is an extremely under-researched area, even in terms of the well illustrated regalia of the rulers, let alone of the lesser members of the court which members of the Hetaireia will more commonly occupy.
A few publications presently available give a little information:
Very much more will be accessible with the publication of Dr. Timothy Dawson's By the Emperor's Hand: court regalia and military dress in the Eastern Roman Empire by Frontline Books in 2011. The following picture gives a sample of the contents. It represents three junior courtier in the most basic regalia. The man on the right holds the rank of Disupatos, as symbolised by the sealed scroll. The other two might hold any of several junior ranks, but fulfill a function in the court represented by the emblems they hold. The red baton marks one as a Mandator, something in the manner of a Western Town Crier. The gilded whip places the other as a officer of the imperial stables. The cloaks they wear are interchangeable, depending upon occasion.
For various examples of clothing for both men and women, some of which touch upon regalia, consult the clothing pages of Levantia. The page will load into this window, by permission.