First, this means _club_ , _group_ or _society_ rather than an individual _meeting_. Second, there is no word for _if_ in your sentence. Third, is _various activities_ , and there is no word for _exciting_ in your sentence. Fourth, you have already asked about this A construction (or more broadly `AB`) several times. `AB` is a very important pattern and you can read dozens of questions about it here. is adverbially modifying .
Without this part, the sentence should be easy:
>
>
> This group is working on various activities.
And with the A part:
> ****
>
> (literally) This group, **with the chairman Mr. Kimura at its center,** is working on various activities.
>
> This group, led by the chairman Mr. Kimura, is working on various activities.