1. Fixing passive voice
There is a considerable range of expertise demonstrated by the spam senders.
send the weak “there is” opening packing
The spam senders demonstrate a considerable range of expertise.
It was determined by the committee that the report was inconclusive.
Again, the subject is weak and indeterminate. Two actors, the committee and the report, are vying for the lead role here, but committee is the bearer of the news about the report, and to place the report the head of the sentence would be to replace one passive sentence with another. Attend to the actors:
The committee determined that the report was inconclusive.

2. Fixing passive voice
We were invited by our neighbors to attend their party.
We is stronger than it as a sentence opener, but “our neighbors” is stronger still:
Our neighbors invited us to attend their party.
The proposed initiative will be bitterly opposed by abortion rights groups.
The content may be about the proposed initiative, but that doesn’t preclude given a sentence about it a more dynamic structure:
Abortion rights groups will bitterly oppose the proposed initiative.