In the rule Imaginary Friends, append the following to the end of the first paragraph:

Each Imaginary Friend is tracked in a separate row on the GNDT.

In the rule Calvinball, replace:

Possession is tracked in the GNDT field “Ball”: if possessed by a Child, that Child’s GNDT field reads “Me”; if possessed by an Imaginary Friend, the GNDT field of that Friend’s Best Friend contains the name of that Imaginary Friend.

with:

Possession is tracked in the GNDT field “Ball”: if possessed by any Child or Imaginary Friend, their GNDT field reads “Me”.

Enact a subrule “Teams” to the rule “Calvinball” with the following text:

Each Child and each Imaginary Friend is on a Team, tracked on the GNDT under “Team”. Any Child or Imaginary Friend is a Teammate of another Child or Imaginary Friend if they are on the same team. If a Child joins the game or a Imaginary Friend is created, they are assigned to the team with the fewest members, or, in case of a tie, to a random team among those tied for the fewest members.

Of all the Children and Imaginary Friends randomly select half of them, rounded up, and set their “Team” to “Red”. For the rest, set their “Team” to “Blue”.