Hi @David_Hess1,
@Bastien_Siebman is correct: while you can’t accomplish what you want in the current API, there are changes coming literally any day now, which are explained here:
The bottom line, from my understanding, is that you’ll be able to add a user to a project with write access by doing a POST like this:
{
"member": "<User or Team resource>",
"parent": "<membership-capable resource>",
"resource_type": "membership",
"resource_subtype": "project_membership",
"standard_access_level": "editor",
"<subtype-specific fields>": "<subtype-specific values>"
}
This is derived from @sasha_f’s comment here in that thread.