If you know what people or accounts you’d like to follow, open the sidebar. Then go down to search and open that. There will be a search field along with two tabs, which allow you to switch between searching for tweets and searching for people. Make sure the people tab is selected. Once you find a user and open their profile, there will be a follow button at the bottom left corner. This profile page, along with the follow button, is the same no matter how you come across a user, so if someone you follow mentions someone else, you can just open their profile and follow them from there rather than needing to search.
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If you know what people or accounts you’d like to follow, open the sidebar. Then go down to search and open that. There will be a search field along with two tabs, which allow you to switch between searching for tweets and searching for people. Make sure the people tab is selected. Once you find a user and open their profile, there will be a follow button at the bottom left corner. This profile page, along with the follow button, is the same no matter how you come across a user, so if someone you follow mentions someone else, you can just open their profile and follow them from there rather than needing to search.