Which plan does work to follow collaborators' activities in a workflow?

[Mercedes Ponce de León]

1. If I pay a premium user, can other non-premium users interact?

2. How many users can be on a board with free accounts?

3. How many users can be on a board with a standard and premium account?

[Sagar Rao]

Hey There.

1. Yes, you can add Guests to board where you would not have to pay for them as long as they are just part of one board. You can read more about Guests here - https://support.atlassian.com/trello/docs/multi-board-guests/ 
2. You can add up to 1600 members on a single board.
3. The limit is the same for all plans including Standard and Premium. You can add up to 1600 members.

Thanks!