This is just another case where you need aggregate data (sum, count, average...) from the related records in a portal. There are two methods you can use. Both give the same result but the design of your database and use of a given layout may argue in favor of one over the other:
Define a calculation field in the Committees table as Count ( Committees_Assignments::CommitteeID ) and it will present you with a count of the committee members listed in the portal. A similar field in Members would count a member's committee assignments.
Define a summary field in the join table as the "count of" any never empty field such as either of the two ID fields. If you place this field on your committee layout, it will give you a count of committee members. On a member layout, it will count the number of committees.
The summary field approach allows you to add one field to use for both counts, but can have screen refresh issues when you edit records in the portal in a way that should change the count displayed. Thus, with the summary field, you may need a script trigger controlled script to refresh the window each time you add/remove a record in the portal.
Not sure where I should post "Thank you!" since this box says "Post a Answer" (shouldn't that be "Post AN Answer?"), but I don't see another place for posting. So thank you, Phil! I will give this a try as soon as I have a chance.