Learn English – “Covering letter” vs “cover letter”

the people is commonly used in our workplace.

Call up the people in [department name].

Someone is also used

Can you get someone in accounting to take a look at this?

Or, it's just omitted if it's a whole team:

Can you get Finance in here right now?

EDIT: I should add that if a team is all young men, sometimes the term boys is used, though this is usually used within a team and is much more casual.

E.g. The IT(sub-dept.) Manager might say to others in the IS(main dept.) team: "The boys are out fixing a problem, they'll join us shortly"

In the general sense, if the letter is a general letter from your mother's employer confirming that she works/worked there and that her work was satisfactory (or more than satisfactory), it would be termed as a reference (usually plural references). If, on the other hand, it literally confirms only the fact of her employment, and possibly the conditions of that employment, it would be a statement of employment or letter of employment.