If so, then why didn't she fill us in immediately on her team when making the offer?
Because the information sharing is a part of the deal, according to Redshirt Army, not a prerequisite for it?
When/if we do end up fighting Walpurgisnacht together with Fukushima girls, we ought to tell them about our powers a
lot, and vice versa, because killing her without anyone dying will be hella hard. Let me put it that way: Walugi is a plot devise with a sole purpose to make Homura suffer. Just because she's found a few friends and that weird girl helping her looks just like a giant spider, if you squint, doesn't mean she's going to give up easily. Or at all.
Right now, if I understood it correctly, RA offers just to name the powers of Mitakihara Megucas.
Like
"Homura is a timestopper."
"Oriko is a bullshit precog."
"Being Sayaka is suffering. Also, power copying."
It's actually less than what they would find out about them if they just call up Kyubei and ask
"What's the Girl X's deal?"
Again, Homura didn't tell anyone about time stop not because it would give her an unfair advantage against other Magical Girls they will not be able to counter as they don't know what to counter and how, but because she's emotionally broken teenager stuck inside a month-long loop where everyone who should know better ( magic! ) ridiculed her warnings, and then died. Or went crazy and then died. Or sacrificed herself to save her, and then- you get the idea.
If Homura
is uncomfortable with disclosing anything about her powerset, then she can tell us so. She could then provide us a belivable lie that she must've thought of in her eight years.
She is unlikely to be hurt by the very mention of the idea.
Logic as follows: one side offers a mutual defense pact. The other tells that the pact will be likely weighted in their favor, because in less than a month they will almost certainly get attacked by the thing Witches have nightmares about. The first side is still willing to follow through with the offer, but first offers a spar with the perceived leader of the group in order to gauge her power levels.
In this scenario, telling about other members' powers is a logical step, basically advertising what two sides of the deal have to offer each other. They can find the other way. We have Homura, they have a Kyubei. This is inconsequential.
Homura looks and feels vulnerable to us, because we know exactly what kind of shit she went through. To many others, she is largely a pragmatist who set out to accomplish one goal, i.e. defeat Walpurgisnacht. She can and will act the part to acquire assets. It's not like she hadn't told about her powers to anyone before, you know.
Eh, whatever. Redshirt Army, looks like you're outvoted.