My activities is two things on one page: a stream of what the people you follow have been doing, and the three lists that describe who you are connected to.
Open the menu under your name and choose My activities. The stream is built from the people you follow - their RSVPs, uploaded images, username changes and profile pictures. Your own RSVPs and follows are not in it, despite the introduction on the page saying otherwise; what does appear from your own account is a profile picture change and the blocks you have made. It is a record rather than a feed to interact with: there is nothing to like and nothing to reply to.
Following starts in the member directory
Members in the top bar lists everyone who is visible, and each member page carries a Follow button beside Message and Block. If you already know someone's username, the page suggests the other route: ask for it at the next meetup.