Is there a way to automatically add a user as a participant on a news feed entry

Users can be added as participants manually as described in the documentation linked above. They are also added as participants when they comment on the entry. If you know all the users who you want to add as participants and have them stored in an organized way (a list of users or usernames, for example), you could automatically author a comment from each one to add them as participants. (e.g., "User 1 has been added as a participant.") This could make the entry look a little messy, however, if you want to add a large group of users as participants; you might end up with a lot of comments that clog the entry feed.

Users can be added as participants manually as described in the documentation linked above. They are also added as participants when they comment on the entry. If you know all the users who you want to add as participants and have them stored in an organized way (a list of users or usernames, for example), you could automatically author a comment from each one to add them as participants. (e.g., "User 1 has been added as a participant.") This could make the entry look a little messy, however, if you want to add a large group of users as participants; you might end up with a lot of comments that clog the entry feed.