I'm very slim. Does this mean I'll have more difficulty building muscle?

My body type is an ectomorph, and I've heard from many sources this body type usually has difficulty building muscle. Is it true for all ectomorphs? I'm going to join the gym soon to increase muscle mass on my legs - will my slenderness hinder my progress? I also have a fast metabolism if that makes any difference.
I'm very slim. Does this mean I'll have more difficulty building muscle?
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