If you have been doing your document editing offline, and are realizing how much more difficult it is when working with a team, then looking for a site that allows markdown online is key.
The problem comes in when you realize there are thousands of sites offering markdown online, which makes it difficult to choose the right one for your needs.
Simple is key -- Look for simplicity and ease of use above everything else. As yes, it is nice to be able to do complicated things, but when even doing the most simple of things is complicated you will get annoyed quickly and move on to something else.
Ease of collaboration with your team -- If you are looking for a company offering edit markdown online, you are probably also looking for one that allows you to collaborate with your team.
Choose a company that makes inviting your team and getting them online easy, and that has a site set up that makes editing a breeze.
Exporting to markdown -- If you are going to be writing your document online, you will also want the site you choose to have an easy exporting to markdown function.
After all, if just getting a document into the markdown function is a hassle, how likely are you going to want to edit it?
A site that has an easy exporting to markdown function usually makes the entire process easy as well.
A free CDN -- If you want to be sure you have the security you need, then finding a site offering markdown online that also has a complimentary CDN is mandatory.
Not only will that CDN give you the strict level of security you want for all your work, chances are it will also make everything you do much faster as well.