If you want to come, you're welcome to come with us.
Literal
Come-want-[explanatory-のなら] we [with-と] together come-[even-ても] good-is [emphasis-よ].
のなら (explanatory + topic-conditional) = 'if it's the case that.' 一緒に ('together') + と marks the companions. ~てもいい is the permission form 'you may / it's OK to.' Warm casual invitation.