A recent CBC news article has explained how the rules for official party status in the Senate have now changed.
This has been in the works for a while, with the creation of the ISG Crossbencher group, and the granting of additional power to Independent Senators.
The former rule stated that your party needed one of two things.
1 - To already be an official party in the Senate.
2 - To have 5 Senators in your party, and, have a party that ran in the last election.
This meant that only the Liberals and Tories were ever official caucuses in the Senate, as no other party qualified. Theoretically, had the PC Party maintained an unbroken line from 2003 with more than 5 Senators, they too would have qualified; but the party dipped below this number.
The new rules are simple, you need 9 Senators.
This is much more in line with Commons rules that state 12 members are needed for official party status, regardless of what that party is. 12 Independents could group up to form a caucus even if they have no official party.
9, however, is a fairly high number. As a share of seats, the same number in the Commons would be 29, not 12. The number seems to have been set as 1/12th of the seats in the Senate, which, makes sense, as, being an official party gets you a lot on committee, and many committees have 12 members. 1/12th of the Commons would be 29.
This also prohibits certain things from happening. Only 4 provinces have more than 6 Senators, Ontario, Quebec, New Brunswick, and Nova Scotia; so none of the remaining provinces, on their own, could create a provincial caucus, however regional caucuses could be formed.
We could also see caucuses created for other purposes. For example, an "Amendment Caucus" could be formed by those senators who feel the main purpose of the Senate is to amend Commons bills to be, in their minds, better. We could see an "Urban Caucus" formed to represent the cities, and, a "Rural Caucus" for the rural areas; these are cross-province issues.
In the short term, this will officially give full recognition to the ISG, but in the longer term we may see more political movements that cross party lines, such as the formation of a general-left "Progressive Caucus"