1. Begin the program by including the header file <iostream> which includes basic input/output library functions as well as the <vector> library which is needed for this program.
2. Declare a vector of integer type named 'ageGroups' which will store the age groups and the corresponding membership fee and reductions.
3. Create a void function named 'calculateFee()' which will take a parameters age and activities attended.
4. In the calculateFee() function, use the switch statement for the age group and store the corresponding membership fee and reduction value in variables.
5. Use an if statement to check that the number of activities attended is non-negative.
6. Calculate the membership fee using the variables and store it in a variable named 'fee'.
7. Use an if statement to check if the fee is less than 1 and if yes, assign the fee to 1.
8. Print the fee to the user.
9. End the program.