Campbell and other big name artists (Hughes, Jim Lee, etc), usually have you leave the comics with CGC and then sign them all privately. CGC will notify you if that is the case when you get to their stand. Many of the other artists or actors, CGC will send a witness with you. If the line is too long, the guy may give you his number to text, like it was mentioned above. The CGC crew have done this before and have plans in place. Other groups may have their own witnesses taking care of things too at their booths, Comic Sketch Arturo example.
Basically, find the CGC booth and they will set you up!