We definitely know the identities of three others who join the kids in the Realm during the series (although it's still a very exclusive club):

Josef Mueller was an ace pilot for the Luftwaffe before being lifted out of time by Venger (see "The Time Lost"). We have DM's word for it that he pulls a Rudolf Hess and defects to the Allies. (We also see a few other prisoners, including a viking and some Asian warrior.)

Jimmy Whittaker seems to have known most of the gang before the events in "City on the Edge of Midnight", when Eric reminds everyone of his opinion of Jimmy. (Question: how could Jimmy write it off as a dream when his dad freaked and called the cops?)

Terry (No Last Name Given) is the eponymous "Girl Who Dreamed Tomorrow", and quickly becomes Bobby's best friend and then some (her giving him a heart-shaped locket is major symbolism).

While we're on the subject, a look at love in the Realm would also include:

Varla the illusionist who inspired Presto in "The Last Illusion"; and

Kosar who did ditto for Diana as "Child of the Stargazer". By the way, is it me, or does Kosar bear a resemblance to 1984-vintage Michael Jackson?

Oh, yeah--and Eric was prepared to marry the Queen of Zinn, but somehow I don't think his heart was in it.