Yes, I would consider implementing a helper object to avoid a many-to-many relationship and ensure good long term performance. The helper object will have a many-to-one relationship to member and project. I suggest reviewing the Design Advance Domain model path:
Also, in the Salesforce world, this is typically called a junction object. Googling “Salesforce Junction Object” will return some helpful explanations and the concepts are applicable to Mendix. There’s also a Salesforce Trailhead project that features a junction object:
Here is a start for the domain model:
But there are many models for it. Often you also have an entity called ‘Activity’.