What do you call someone who's involved in a project (non-leading role)

"Team member" is far and away the most commonly used term in business for what you are suggesting.

(See as a reference books by any well-known business leader, such as Stephen R. Covey, and you will notice the commonality of the term, implying that its meaning is widely understood.)

A project would have:

1. A Project Manager
2. Fellow Members (or simply) 'Associates' (or) 'Fellows'

If you are against using 'team member' as a designation, I think the above terms should suffice as a clear, simple way to suggest hierarchy while denoting each person's interest in and commitment toward the project.

Other alternatives:

• Project Co-workers
• Project Peers
• Project Partners
• Collaborators
• Affiliates

These are all good terms to denote, as you say, a 'stakeholder's' position in a project.

If you wanted a more-casual, less-formal term, you could try coining one yourself (tailor-made for your particular project) and it should work just fine - Example: Project Mates

What about "delegate"?

Does that work?

Delegate noun

delegate; plural noun: delegates

/ˈdɛlɪgət/

1. a person sent or authorized to represent others, in particular an elected representative sent to a conference.

"congress delegates rejected the proposals"

Synonyms: representative, envoy, emissary, commissioner, agent, deputy, commissary; spokesperson, spokesman, spokeswoman; ambassador, plenipotentiary; messenger, go-between, proxy; depute; nuncio; archaiclegate

"delegates from the UN"

2. a member of a committee

verb

delegate; 3rd person present: delegates; past tense: delegated; past participle: delegated; gerund or present participle: delegating

/ˈdɛlɪgeɪt/

1. entrust (a task or responsibility) to another person, typically one who is less senior than oneself.

"she must delegate duties so as to free herself for more important tasks"

Synonyms: assign, entrust, give, pass on, hand on/over, turn over, consign, devolve, depute, transfer

It depends on the role of the members in the project.

• Use the same job titles, such as Developer, Artist, Quality Assurance if the team member is solely doing one job or one type of task in the project.
• Team member if the person doing it is not assigned a specific role.
• Current job titles with prefix 'Lead' - Lead Developer, Lead Artist, Lead Modeller, Lead Environment Artist, for supervisory roles one level below Project Manager.

Team member is the term typically used in projectized organizations unless they have an elevated level of authority. See, for example, this page from the Project Management Institute.

How about a "participant" or a "consulted".

Personnel: "1. persons employed in any work, enterprise, service, establishment, etc. see also materiel"