I would like to know if I can advertise my shop in my siggy. Thanks.

Also, if I can advertise my friend's shop, too.

I don't see why that should give any problems. :)

I don't think there's any rules against advertising for shops other than your own, but there most certainly isn't one against advertising your own shop.

Just make sure it's PG13 and within the size limits if it is an image :)