"여러분 and 여러분들"
There is really no difference between the two as far as I know.
여러분 is already plural as you mention, but 여러분들 is also heard sometimes, especially when a specific group is mentioned. You might say it is redundant, but it doesn't sound particularly strange either. It can be considered a way to add an emphasis.
Such usage depends on personal preference and habit more than anything else, as there is no rule about it.
Generally, the plural suffix 들 is only needed when not using it will make the meaning unclear.
Korean has no requirement on agreeing "number" between different parts as English does.