I'm an engineer with over a decade of work ex in the industry. I am thinking about learning a musical instrument as a beginner.

The primary purpose of music is to gain personal satisfaction, but would it also help one in one's career resume in any way, for someone at my stage in work ex? I will be another reason to motivate myself and justify undertaking the immense amount of effort it would take on my part to begin learning an instrument.

It's known that being a member of school marching band may help in top college admissions and a student can concretely cite it as an accomplishment on their resume.

However, what kind of community bands (if such a thing exists) can a working professional starting as a beginner be involved in in a formal role so that he or she could cite an accomplishment related to a musical instrument? If so, what level of expertise with the instrument will be needed for such a role? I am located in US.

I don't know about playing opportunities in the US but I would suggest that the primary reason for learning an instrument bis that you want to learn it. Any other potential benefits are way down the list and without the genuine desire to learn you may well find that you fall by the wayside pretty quickly.
Learning an instrument is a lifetime project.
How good you need to be to join a band will depend on the bands available in your area. Some may have training bands for relative beginners while others could ask for a fair degree of competency.

I agree.

I think for students, new graduates that extra curricula activities are a useful way for employees to assess skills in the absence of actual work experience. But these become less important the more work experience you have. I suppose that it is good to demonstrate that you are well rounded person, able to learn new things etc but in reality I don't think it will really make much difference to your resume. It maybe different in the US but I applied for a new engineering job after about 10 years experience and they were really only interested in my relevant work experience rather than anything I did outside.

It takes hours of practice to be able to play an instrument to even a low level of competency, so if you are not learning because you love the instrument, forget the year or so it would take you to get to training band standard. I agree with everyone else that it may be something looked at when you apply to university to show you have varied interests and achievements, but at your level will not be important. You don't say which instrument you are thinking of playing, but they are all quite expensive, and then you need to factor in the cost of lessons, and the cost of reeds (if needed) sheet music, music stand, servicing...the list goes on.
So, unless you have a great commitment, don't do it!!!!!!