I assume that this HAS to be a command line interface?
Then first off:
Why both "add" and "bookmark"? You should construct the commands with a verb/action as the first part. That is either:
Then the trick is to find som memorable identifier. If you've ever tried using the filesystem on unix you will know that you can get autocomplete suggestions by typing the first letter(s) and pressing TAB (if I remember correctly ;) ). Then you just keep TAB'ing until you reach the desired suggestion. You type the path separator (a slash) and then you can autocomplete the next part of the path.
One suggestion for a memorable identifier could be username/NameOfEntity. Lets say Michael245 has entity 12309453518123. He has named it 'stuff', making the command:
The user could then enter 'bookmark mich', TAB through the Michaels and Micheles, enter '/st', TAB through Michael245's entities starting with 'st'.
This only works if Michael245 is forced to select a name for his entities that is unique among his collection, and if the bookmarking users has some way to explore the entities to discover who ownes what entities - but then the interface wouldn't make much sense without that ;)