If you are asking about someone who wasn't in your group when you qued up for Azura's, and they want to be in Azura's with you (but are committed to two other campaigns as 'main' and 'guest') then they are asking you to "piggyback" them in.

That means that crown (or someone who is willing to leave Cyrodil is given crown of your group) leaves Cyrodil, then invites the person who can't que into Azuras to the group, then re-ques the group for Azuras. The two others in Cyrodil ignore the incoming group invite to Cyrodil. The other two accept and re-load into Cyrodil/IC. Crown can then be given back to the leader of the group, if need be.

I hope that helps and I wasn't explaining some ridiculous process that you didn't mean!