There are several reasons one cannot join a group which need to be taken into account when display search results for a particular quest:

  1. Is the party already full?
  2. Is the party on a block I can join?
  3. Is the party in mission on a quest which does not allow joining in mission?
  4. Do I meet the join criteria?

If any of these is not correct you should hide it from the search results. I don't know how many times I have tried to join groups only to have it give me an error or message return me to the lobby. I know there is gray text when a party does not meet the criteria and an indicator for block capacity, but why even worry about these UI elements if you can just exclude those from the list? It will save people lots of time looking at the UI to determine if they can join.