KB,
You do good missions.  They've seemed doable when I've joined.  I'll join again.  
To answer your question, if I'm inflight and not landing soon, I won't join.  If I tower sooner than I thought, or rtb is more attractive than getting ganged, I'm in.  
If there's a fun furball going on, will I leave it for a mission?  That's a reputation/trust thing that comes with time.  Because I like your missions, it's a definite maybe, not a no.   
I joined ET's and Artlaw's missions because they were well-planned and usually productive.  There's a chance any mission will be busted by the enemy.  So, build them and they will come.