Well Adriana isn't too likely to know anyone other than Monica and Cassandra even if she has already been there for several weeks as opposed to just one or two.  Certainly not very well at least, so she would almost have to have been either invited by the principal, one of the switch students under the guise of just meeting up with some girls to try to make some friends, or brought around by the Hemmills.  Either option is entirely acceptable, but it could be kinda fun if someone other than the Hemmills invites her so she can meet up with them rather and get herself a nice little shock.