General:
* Participant: This is the most common and neutral term.
* Partaker: This implies a more active and willing participation.
* Contributor: This emphasizes the role of the person in making a contribution.
* Involved: This is a broader term, suggesting involvement in some way, not necessarily direct participation.
* Player: This is often used in a more informal context, especially when referring to a game or competition.
More specific:
* Actor: This is used when someone is actively involved in a situation or event.
* Agent: This implies a person who acts on behalf of another.
* Member: This refers to someone who is part of a group or organization.
* Stakeholder: This is used for someone who has a vested interest in something.
Informal:
* Joiner: This is a casual term for someone who likes to participate in things.
* In on the action: This is an informal way to say someone is involved.
The best word to use depends on the specific context and the nuance you want to convey.