Czenglish: We are 30 in our office.
English: Everyone is 30 years old?
Czenglish: No, 30 people work in our office.
English: OK, in that case, there are 30 of you in your office.
When counting the number of people somewhere remember to use ‘there are‘. Context usually makes the meaning obvious but when you say something like ‘we are 15‘ you are technically referring to age, not number of people.
You should say ‘there are 15 people‘ if you are not included. If you are in the group you should say ‘there are 15 of us‘.