Hi @MongDev,
I would say this is possible. You could use a list group members action and compare that with the toRecipients field of your flagged email.
In my example below I am converting both to arrays and use an intersection function to compare the two. The result is converted back into a semi-colon separated string, because this is required for the assigned to field of a planner task.
1. Add an Initialize variable action and use a split expression to create an array for the toRecipients field from your flagged email.
split(triggerOutputs()?['body/toRecipients'], ';')

2. Add another Initialize variable of type Array, call it for instance PlannedUserEmails, leave the value empty

3. Add a List Group Members, select the group of your Planner Plan

4. Add an Append to array variable action. Set the value to Mail for the PlannedUserEmails variable. This is a dynamic value from the previous List Group members action

5. Add an Compose, use the following expression to compare the two array variables with an intersection
intersection(variables('FlaggedEmailUsers'), variables('PlannerUsersEmails'))

6. Add a Create the task action. Use the output of the compose for the Assigned users, by using the following expression
join(outputs('Compose_-_Intersection'), ';')

Hope this helps a bit?