In general, every creature has its own turn
From the combat rules:
Initiative determines the order of turns during combat. When combat starts, every participant makes a Dexterity check to determine their place in the initiative order. The DM makes one roll for an entire group of identical creatures, so each member of the group acts at the same time.
If a tie occurs, the DM decides the order among tied DM-controlled creatures, and the players decide the order among their tied characters. The DM can decide the order if the tie is between a monster and a player character. Optionally, the DM can have the tied characters and monsters each roll a d20 to determine the order, highest roll going first.
While creatures may act at the "same time" they each act on their own turn, the order of which is decided by initiative and the tie-breaking procedure above - tie-breaking is done once at the start of combat, not each round. This covers mounted combat as well with the order of the mount and rider decided at the time of mounting.
The Marilith can react on each and every one of those turns tied or otherwise.
The exception: shared turns
With respect to the specific exceptions to this rule that you linked (True Polymorph etc.); there is only one turn shared between several creatures.
The Marilith can react once on shared creatures' turn.
The Conjure spells ...
They are not sharing turns. This is evident from the use of the same term, "group" in both the initiative rule and the spell description.
What we have is some poor grammatical choices by the editor possibly because the first part of the sentence has both a plural noun - "creatures" and a collective noun "group". This creates some awkward subject-verb (dis)agreement problems.
The editor has gone with a singular (pro)noun - "its" and a plural verb - "turn". The grammatical construction that best fits is that this is a reference to the "group" (since a plural noun should have a plural verb "their own turns") and it places the emphasis on the group collectively rather than its members individually.
So, you have a choice:
- Its poor editing and proof-reading by WotC (which happens) and each creature has its own turn, or
- They mean what they say and the creatures share a turn.
The Marilith is waiting to react based on your decision.