Learn Italian: Sono sicuramente in ufficio adesso. - They are definitely in the office now.

It comes from the adjective sicuro (sure or safe). To form an adverb with the -mente ending, you first change the adjective to its feminine singular form (sicura) and then add -mente, giving you sicuramente (definitely or surely).

How do we know sono means 'they are' instead of 'I am' here?

The verb form sono is used for both io (I am) and loro (they are). Without a subject pronoun, you usually rely on the context of the conversation to know who is being talked about. In a standalone sentence like this one, it could technically mean either!

Why is it in ufficio rather than nell'ufficio?

For certain common everyday locations, Italian simply uses the preposition in without combining it with the definite article (the). In ufficio is a set phrase, just like in banca (at the bank) or in classe (in class).

Could I put sicuramente at the very end of the sentence?

You can, but placing it immediately after the verb is the most natural word order in Italian for adverbs of manner. Putting it right after sono naturally emphasizes your certainty about the situation.