This is a beautiful effort at flagging, which is a competitive strategy in ecological systems. (Also known as branding, logos, etc.)
>> Since the people did not actually try to impersonate Best Buy employees, could they be charged with any crime?
Under the group's operating procedures, no. The managers can trespass them once it's been established that they're not there to shop, but that's about it.
I would have really enjoyed watching something like this if I were working security at the store. Once I established that they weren't present to steal, it's kind of a fun break from the normal routine.
The classic solution to this problem (for example, if terrorists thoughtfully bring their own black BDUs and SWAT vests to the incident) is to pass out a brightly colored scarf or bakclava to each legitimate employee just before the change in operational tempo.
"May I have your attention please? Best Buy shoppers, please be aware that our employees are wearing yellow scarves today as part of a special promotion. Thank you for shopping Best Buy today."
A related technique is to keep a supply of lanyards and transparent badge holders, with a word processing template so that you can print the Word of the Day and the date. A Staples, Office Depot, etc. could do this with off-shelf items if a manager thought fast.
"THANK YOU FOR SHOPPING BEST BUY. 5/4/2006."
We have had occasional thoughts about security personnel wearing fake uniforms, especially because badges etc. can be purchased on eBay. A quick answer is to ask a question that only a legitimate member of the security team can answer.
"Who is the regional security manager? Where is the master fire panel? What is the radio code for officer needs help?"
A better, classier answer is a discreet pin of the day -- week -- month -- or best of all, per special event. This is what the diplo types use.
(Dallas PD is still embarrassed about the time they left the box of Secret Service Secret Pins in an unsecured suite and not just uncleared police officers but reporters managed to lay hands on pins.)