I think "assemble" normally used as a mechanic term (meaning put the parts together), and rarely used with people, right? In this case, you can you "come together" or "go together".

assemble = put something (someone) together ,so I think together here is redundant

"Let's assemble at 9am." This has a formal and organised implication, so you would be talking about a school assembly, a military operation or a fire drill. The word "together" is redundant.

You can, but I wouldn't bother saying "together" as assemble basically means to come together.