US to Canada to attend meetings/training

[HR]

Hello....... hope you can help!

There is a small team from the US coming to Canada to attend meetings and training for approx. 4 weeks.
Some of them have H1B visas.
What do they require to enter Canada? 

Also, for those that are US citizens, will invitation letters from our side suffice?

(Please note, it is all the same company, just different locations)

[SerenityNow]

Everyone needs a passport. US citizens do not require visas to enter Canada for less than 6 months. For the others, check below if they need a visitor visa. You may want to make sure their US visas are still valid to allow re-entry.

http://www.cic.gc.ca/english/visit/visas.asp