From the book we know [HoHMC p76] that Merinita joined the founder in 767, and that she became close to Birna and became her mentor and that they devised together the Ritual of Twelve Years [HoHMC p8]. It is not clear when the first Ritual of Twelve Years was held, but there was one in 831, we can suppose that the previous ones were in 819, 807, 795 and that the first one was in 773 at the founding of Crintera [GotF p19].
Birna took her first three apprentices from her tribe: Midusulf, Maruhs and a third unnamed one who was a traitor and killed by Birna [HoHMC p8]. Only after killing this one, she started to look from apprentices outside her tribe. Therefore I suppose that during the first Gathering at Crintera in 773, only those first three were initiated to their hearbeasts, and that the next apprentices (Ilfetu, Wilkis, Arelie and Sirnas) were initiated later. There is also the case of the few followers of Myanar who joined house Bjornaer after the defeat of Myanar in 802. I supose that all of them were initiated in 807.
I would assume that at least Ilfetu and Wilkis were initiated in 795, and that both of them had apprentices initiated in 807. There might even be some apprentices of Midusulf and Maruhs that were initiated in 795.
Do we have any idea of how many followers of Myanar joined house Bjornaer? The rulebooks use the plural, meaning that they were at least two, but the numbers of members of each house were rather low at the time, so I would say only two or three.
When I did my Hermetic Calendar, I went back from the last known Gathering, and it came out to the first gathering being held in 771.
Indeed... so it would mean that there was a gathering in 783 in Crintera, but that the first Ritual was in 771 before the founding of Crintera, maybe with only Merinita performing the ritual on Birna.
Or, it could be that Birna performed it on an apprentice in 771, at the site where Crintera is located, but the actual founding of Crintera as a covenant came about 2 years later.
Some other remark, related to the early times of the Bjornaer house.
HoHMC describes a lot of mysteries, and we know that the first mystery was discovered by Birna and Merinita in the early 770s. But I don't think that we have any information on when the other mysteries were discovered.
The Secret Name mystery is likemy to have been discovered early, maybe at the same time but Birna did not change her public name.
Sensory magic was invented by Birna and Bonisagus [HoHMC p27] so it was also discovered early.
No information of when Theriomorphy appeared, but I would expect that it was discovered early, because early Bjornaer were from traditions of shapechangers.
Inner heartbeast is different, I would expect that it was discovered later, and that Birna only knew of the Mystery of the Epitome (or she might not even have really known this mystery, but I think that when she left in bear-form in 831 she transformed into a Bear of Virtue).
Clan initiations probably appeared much later, beginning of the 10th century, when Bjornaer clans started to have traditions.