Multiple ways but the built-in method is to go to the person's profile, on the right drop menu, choose Give Llama and follow the prompts. It's free so when you have already given one to someone, then you can't give any more later.
There is also a script, depending on how technically savvy you are that can make the process easier and avoid having to go to each profile one by one
Let me know if you are interested and I will point you to it.
Let me know if anything else.