Park policy is if you want to ride (watch a show, etc) together, you need to stay in line together. At TITW for Nemo, if you're caught "holding" for the rest of your party, you'll likely be asked to join the end of the line (and probably not get in, or at best get in standing room only). Note the keyphrase "if you get caught". I don't know how they do things over in Camp Minnie-Mickey, but in Dinoland they usually keep a pretty sharp eye out for queue jumpers, etc.
Great example is this, and it happens pretty frequently: People getting in line for Nemo come to a cast member and say, "the rest of my party (usually my kids) are already in the theatre." Policy is, if you want to watch the show together, you need to wait in line together. Security can be called to retrieve the rest of your party from inside the theatre and everyone goes to the end of the line. Of course TITW is a much different scene than Lion King... it seats more, there are people in almost every show who do not get in, etc. I suggest either rejoin your party before the doors open (as others have said) and hope nobody catches you, or just stay with your party and bring something to entertain everyone with in line.