Are the jinn who met the Prophet (blessings and peace of Allah be upon him) and believed in him regarded as being among the Sahaabah?

Yes, they are regarded as being among the Sahaabah, because the definition
of a Sahaabi, according to the scholars, is someone who met the Prophet
(blessings and peace of Allah be upon him) as a believer in him and died
as a believer. 

The jinn met the Prophet (blessings and peace of Allah be upon him) and
believed in him, so they are included under the heading of the Sahaabah.
Allah, ma y He be exalted, says (interpretation of the meaning):

“Say (O
Muhammad (blessings and peace of Allah be upon him)): It has been revealed
to me that a group (from three to ten in number) of jinns listened (to this
Quran). They said: Verily! We have heard a wonderful Recital (this Quran)!

It guides to the Right Path, and we have believed therein,
and we shall never join (in worship) anything with our Lord (Allah)”

[al-Jinn :1-2]

“And
(remember) when We sent towards you (Muhammad (blessings and peace of Allah
be upon him)) Nafran (three to ten persons) of the jinns, (quietly)
listening to the Quran, when they stood in the presence thereof, they said:
‘Listen in silence!’ And when it was finished, they returned to their
people, as warners.

They said: ‘O our people! Verily! We have heard a Book (this
Quran) sent down after Moosa (Moses), confirming what came before it, it
guides to the truth and to a Straight Path (i.e. Islam)’”

[al-Ahqaaf 46:29-30]. 

Ibn Hazm (may Allah have mercy on him) said: The Muslims did not differ
concerning the fact that among the jinn were some who accompanied the
Messenger of Allah (blessings and peace of Allah be upon him) and believed
in him … those jinn followed the truth and deserve respect from us, and
attained a similar level of knowledge and religious commitment as the rest
of the Sahaabah (may Allah be pleased with them all). This is something
concerning which the Muslim does not have any doubt.

End quote from al-Muhalla (9/4) 

Ibn Hajar (may Allah have mercy on him) said, when discussing the definition
of a Sahaabi: As for the jinn, the more correct view is that they are
included in this definition, because the Prophet (blessings and peace of
Allah be upon him) was definitely sent to them, and they are accountable.
Among them are both sinners and obedient ones. With regard to those among
them whose names are known, there should be no hesitation in regarding them
as being among the Sahaabah.

End quote from Fath al-Baari (7/4) 

And Allah knows best.