When clicking a solarsystem a menu will show up including Custom Mark options amongst other functions.
DOTS appear above the solarsystem node.
NODE mark colors the solarsystem node.
TAG marks outline a solarsystem node.
Own and Shared Marks
Only shared marks would be shared amongst your sharing group. Own marks never get shared. You got 2 possibilities to switch which marks you set and at the same time which ones get displayed on maps. Click "PERSONAL" or "SHARED" in the solarsystem menu to switch or Toggle them within the sharing settings.
You can name marks within Settings. Click the little on the left within the menu to get there. If a named mark appears on a map it will show up in the legend.