Converting word problems to math or arithmetic
I will suggest that you not take phrases out of context, but read the whole 10th level ability to arrive at the answer.
Starting at 10th level, you gain temporary hit points whenever you
finish a short or long rest. These temporary hit points equal your
warlock level + your Charisma modifier. Additionally, choose up to
five creatures you can see at the end of the rest. Those creatures
each gain temporary hit points equal to half your warlock level + your
While it would have been nice for them to provide an example in the text of Xanathar's, which begins with the Temp HP bonus for the Warlock and then addresses the bonus for allies, there are two steps to it. You have to assess
- Your warlock's level
- Your Charisma Mod.
Your first example is the correct answer: (level/2) + (Cha Mod).
Because the intent is clear from the previous sentence that explains the Temp HP for the Warlock under the same ability before it addresses the warlock's allies.
Starting at 10th level, you gain temporary hit points equal to your Warlock level plus your Charisma Modifier.
A warlock with a 18 Cha at level 10 would have 14 Temp HP, broken down into 10(level) + 4(Cha Mod).
That is clearly two different quantities added together. Having established that basis, the text follows with:
Additionally, choose up to five Creatures that you can see at the end of the rest. Those creatures each gain Temporary Hit Points equal to half your Warlock level + your Charisma modifier.
Two separate quantities, added together: half Warlock level added to Charisma Mod.
The Charisma modifier term is unchanged, only the half your Warlock Level is changed from the base benefit for the Warlock. Therefore, five companions of that same Warlock, Cha 18 at level 10, would each get 9 Temp HP.
Compare to a similar feature/feat
There is a feat that provides Temp HP that precedes this feature (it was in the PHB a few years before Xanathar's was published, and looks to have been the template they worked from). The feat gives your allies your level + Cha mod. The Celestial Warlock feature has the same pattern but is not quite as strong, and the feat recharges on a short or long rest.
You can spend 10 minutes inspiring your companions, shoring up their
resolve to fight. When you do so, choose up to six friendly creatures
(which can include yourself) within 30 feet of you who can see or hear
you and who can understand you. Each creature can gain temporary hit
points equal to your level + your Charisma modifier. A creature can't
gain temporary hit points from this feat again until it has finished a
short or long rest.
(FWIW, all three DM's I've had Celestial Warlocks under agree with this reading of the text).