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It's not completely clear from what you say, but I suspect that you're
really asking how to allow developers to create new tables without
modifying or dropping existing ones. The diagram is just an interface
- changing the diagram doesn't automatically change the database,
unless you generate and run the SQL scripts required to apply the
changes (as far as I know - it's not a tool I use myself).

If that guess is correct, then the best solution is probably to allow
the developers to work in a separate development/test database, and
have a DBA script and deploy the tables/procs etc. when they are
tested and ready for production. Alternatively, you could grant CREATE
TABLE to the developers, but then you will have objects owned by
several different people (instead of always dbo), and that would be
very awkward to work with.