I've always helped other mappers with their stuff even during my mod days. It's a good exercise for team work. Also you will benefit from it if the extra polish makes the mod more popular. You also gain respect from your team members and leaders, that's something that can be important later on when they work in the industry and can help vouch for you to land a job at a studio.
I wouldn't put a map on the portfolio if you only made a minor contribution. But when you made a good contribution to the visual appearence of the map i don't see a problem. Just write that you did the art pass for this map and if it's worth it make a before and after comparison.