Marcus's comment from 3 years ago is spot on! Worked for me. So people dont need to scroll so far back:
So, let me present a workaround that I think will save some frustration for you - at least it does for me.
1. Make sure that the User Activation Email for the requester is turned off
2. Create a new Customer - yes, a Customer - not an Agent!
3. Set a password by clicking the Change Password button
4. Click the Convert To Agent button and select what type of agent you want
We're done! And the most beautiful thing; when you convert a Customer into an Agent - no activation mail will be sent out! :)
As a bonus: When you create Customers you can import them from a CSV file, unlike when you create Agents, and that potentially saves a lot of time when adding multiple new users in a batch. Unfortunately there is no way to set passwords or convert Customers to Agents in batches, at least not out of the box and that I know of.