Who can do this?
Any user with Add & Upload permissions in a collection (or is a collection manager) can add media to that collection. Administrators should also refer to Add Media to Collections (Admin)
Administrators or Delegated Administrators who are members of a collection do not have a library page as shown in the below instructions, but can still add media to the collection, either from within the collection itself or from the Collections sharing section of the Media Details page, as described below.

The media shown in your Library can be added to any collections where you have Add media permissions. It can also be removed from any group where you have Remove media permissions

You can add or remove media for a collection in three ways:

From inside the collection
From the menu button on the media tile on your Library
From the Media Details page - those are the instructions included below.

From your Library, click ON the media tile to open the Media Details page. All sharing you are permitted to do with this media is available in the Share Settings section of the the Info tab (the tab that appears on entry).

Library page with navigation identified and showing media tiles for selection as described

Scroll down! COLLECTIONS sharing is located at the VERY BOTTOM of the page.

Info tab of the media details page showing the top and bottom sections stitched together with arrows to indicate scrolling and the Collections area identified at the bottom as described

Adding media to a collection

Once media is added to the collection, all collection members can view it. Some members may also be able to download or copy or post media from the collection to other locations.

You may or may not be able to REMOVE the media from the collection after you have added it; remove media permissions in collections are assigned separately from add media permissions.

To add media to a collection from the media details page

Click on the Media Tile of the item you want to add, to open the Media Details page (shown above).
Scroll to the BOTTOM of the media details page to find the Sharing to Collections section of the page (shown above).
Click in the Share with drop-down list.
The drop-down list opens to show only those collections where you are permitted to add media, as shown in the below figure.
If the collection you want does not appear at first, begin typing the name of the collection where you want to add this media. If it does not appear at all, you do not have Add & Upload permissions to that collection.
Select the collection you want from the list.
If appropriate, repeat these steps to add this media to another collection.

The media is immediately added to the collection, and the collection now appears in the Collections list.

Collection list with selected collection from procedures now showing in the Collections share list as described

Removing media from a collection

The Collection area of the media details page allows you to remove media from any collections it resides in, but ONLY IF you have Remove media permissions in the collection. If you do not have this permission, you will not see the X button for removing the media as described below.

ALTERNATELY, you can remove media from a collection via the Collection page itself if you have permissions to do so.

To remove media from a collection

Click on the Media Tile of the item you want to remove, to open the Media Details page (shown above).
Scroll to the BOTTOM of the media details page to find the Collections section of the page.
Click on the X next to the collection from which you want to remove this media, as shown in the below figure.

The media is immediately removed from the collection, and collection members can no longer view it.