Too Long; Didn't Read:
With an "is," she is being shared, not sharing.
Stripping away everything except the subject and the predicate (verb, in this case an action verb), you have:
Although this begs the questions, she shared what? You wouldn't say
She is shared
Because although grammatical, that would mean that other people are sharing her, not that she is sharing something.
Now that we have our base, we can start adding the other descriptive words in:
She shared in the rent
This means that multiple people paid rent, and she was one of them. In my opinion, just saying
She shared the rent
would sound better, but that's neither here not there.
She just shared in the rent
With "just" added in, this means that either she only paid rent, while others paid more, or that the previous assumption was that she paid the whole rent, and newer knowledge indicates that she shared it. I would need more context to be sure.
Maybe she just shared in the rent
With "maybe" added in, this means that the speaker is not sure of the above (about "just").
Maybe she is just shared in the rent
This is grammatical. However, it still means that she, herself, is part of what is being rented, with the add-ons previously described of "maybe" and "just."