To screen share you might need to contact Familysearch Support by phone (if available from your location):
Note: at the bottom the support calls screen share 'remote connect'.
When there are people in the source that are not a parent, spouse, or child, it takes a little more work to get people to line up. You need to use the blue bar just above the main or focus person spot. Click on the left where it says change and pick the person in the source you need to attach. Then click on change on the right. If the person you want to link to is there, just pick that person. If not, then paste in the ID of that person to pull in that person. Then proceed as usual.
I think David is talking about attaching an unindexed image as a source, whereas Gordon is talking about Source Linker, which only applies to indexed records.
The capability originally suggested at the top of this (ancient) thread -- of adding sources from your Source Box to multiple people -- has now been added, but it is not accessible if you open your source box from a tree profile. You have to go to your source box starting from your account logo at the top right. When you do that, each citation in your box has an Attach button at the right, which allows you to choose profiles to attach to by name, by ID, or from your Recents list. It'll still only do one attachment at a time, but it can be a much more efficient and quick way to do it than navigating to each profile's Sources tab.
What they haven't done is enable one to add (rather than copy) a source from a profile source list to one's source box, or to add attachments to a source when viewing the "Attached To" list. One of these days I'll post a specific suggestion about that.