That is a good question, and I would like to know as well.
My two denarii is as follows:
Obviously Bonisagus was the first wizard that Trianoma convinced to join the (potential) Order.
I think Mercere was a very early joiner, as he traveled extensively with Trianoma finding other wizards. I am tentatively thinking he is number two.
Diedne was a pagan, so I think she would need to be a member before Flambeau, otherwise militant christian that he was, Flambeau may have tried to block her entry. Tentatively call her number 3.
However, when Trianoma convinced Flambeau to meet the other wizards to form the Order, Mercere apparently was not with her. This possibly means Mercere was still sharing knowledge with Bonisagus, before he took to journeying with Trianoma. So calling him number 4 would be about right.
Since Guernicus had to be recruited, it suggests that his search for his master's killer took place before Trianoma started recruiting, or before news of it spread far. If Guernicus was looking for a magical thief and there was news that wizards were recruiting magical knowledge, surely Guernicus would have checked them out. Thus Guernicus might be number 5.
Criamon and Verditus are difficult to place. Bonisagus couldn't fully understand their Philosophy/Mystery Cults after what seems to be lots of study, but still managed to integrate quite a lot of their knowledge of Twilight and Enchanting into Hermetic Magic Theory.
Apparently it took a year of searching to convince Merinita to appear. So Merinita was either very famous and one of the early wizards to be recruited, or several of the other Founders spoke highly of her so Trianoma spent the extra time looking for her, meaning Merinita is one of the last Founders.
And Bjornaer/Birna would have joined at the same time, or later, with Merinita's sponsoring clearing Flambeau's objections of a pagan. Birna/Bjornaer's lack of Latin meant communication was slow between her and Bonisagus.
I get the impression that the League of Thebes were ignoring the Order's forming, until it began to be obvious that the western barbarians might be successful in forming a super-powerful order, so Jerbiton was sent to add some civilisation to the unwashed wizards. Hence Jerbiton was probably a late joiner.
Tytalus and Tremere (with Pralix) killed Guorna the Fetid after she returned from studying with Bonisagus, and they marched on Guorna's "allies". These were probably the last two Founders to join.