In order to @tag a user or company in your post, type the "@" symbol, followed by their Twitter handle in the editor. You can always check if the tag is correct by clicking on it and confirming that it takes you to the correct Twitter profile:

Assembly also allows you to tag users in media (images, videos, etc.). In order to do so, upload an attachment to the Tweet, then select the "Tag in Media" option in order to add any tags.

From there, enter the Twitter handle and hit "Save" for any user(s) you'd like to tag.