Which sentence structure is correct?
The meaning is somewhat ambiguous. Whether a group is treated as singular or plural depends on how it's being interpreted. And it's somewhat different between the US and the UK.
The problem with your sentence is that you treat the group as plural at the start, but then switch it to singular at the end. That's not something you should be doing. Either have the group always be plural or have it always be singular.
But while the second suggested revision of the sentence would probably not be a problem in UK English, it's not quite idiomatic in US English.
There are some additional variations you could consider—the first being a slight change to that second suggestion:
- Proponents of progress, the group members were a force to be reckoned with.
- Composed of proponents of progress, the group was a force to be reckoned with.
The first variation puts the emphasis on the members, which you might not want. The second variation keeps the emphasis on the singular group itself, but in a way that mentions individuals while still keeping the singularity consistent.