Thank you for reaching out here in the Community!
Currently it's not possible to view who follows your playlists.
If that's something you're interested in, make sure to add your +VOTE to this idea.
More details on how your feedback reaches Spotify can be found here.
The circle you see next to every song is a profile of the user that added it the collaborative playlist. The letter is a first letter of user's display name. If a user has a profile picture, it will be displayed in the circle instead of the letter.
I hope it helps!
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Note: I'm not a Spotify employee.