"...they are clustering on the hive entrance as if they don't have enough
room to get into the hive.'  'Should I be concerned by the bunch at the
entrance?"

This is not unusual, and such clusters can often reach very large
proportions.  Sometimes they are called "hive beards".  In my opinion, what
is going on is that the bees do not have sufficient room inside the hive to
ventilate, therefore evaporating the nectar and keeping the hive cool.  I
believe that this clustering also leads to reduced nectar collection and
sometimes (but not always) swarming.

Many years ago I was told that the way to reduce or eliminate these clusters
was to provide plenty of upper entrance room; therefore improving
ventilation.  There are many ways of doing this, including steps as simple
as propping up the outer cover with stones or pieces of wood.  It works,
meaning that with good upper ventilation the beards all but disappear and,
in my opinion, nectar collection increases.

Lloyd

Lloyd Spear Owner, Ross Rounds(tm).  The finest in comb honey production.
http://www.rossrounds.com