RealtimeBoard 2.0 can also be a good fit: simple as Sketch, but collaborative as Google Docs. Check it out here at Product Hunt!
I talked to many Designers using this tool and what they really like 😍
is that it's incredibly simple to share and dicuss your work with clients💰 and developers🔧,
because boards can be commented and viewed without signup🎉.
Although for editing purposes personal account is required.