My subordinates have not been attending monthly meetings for the past few months. How do I write an email to nudge/drive them to attend meetings?
If the meetings are mandatory, then go to the individuals, point out that they need to be attending the meeting, and explain the consequences if they continue to skip.
If the meetings are optional, then a general email about the benefits of the meetings can be sent. But if they still choose to skip, then let them. That's the definition of optional. You can ask if there are things that need to change for them to attend (less work so they have time to attend, more value in the meetings, a charge code to cover the time spent in the meeting), but even if you change that, if the meetings are optional, then let them not attend.
How do you get them to do anything else?
Now, if they're able to ignore meetings because they're actually doing their jobs, you may need to coordinate schedules a little more, setup separate meetings, alter work expectations.
Is it absolutely necessary to have a meeting in person? Is there a history that suggests they're unproductive?
Finally, punish one - teach a hundred.
Write an email to your entire team without pointing to one in particular. Talk about how glad you are that people have attended meetings (point out those who have contributed in the past). Say that the meeting is important so that
<insert metric here>.
Focus on positive first.
If that doesn't work, follow up with individuals (in person) and make sure that you understand why they aren't attending meetings.
Do not yell.