noun
employee, staff member, colleague
A2
Mitarbeiter means employee, staff member, or colleague. It's a masculine noun (der Mitarbeiter) whose plural is identical (die Mitarbeiter). Feminine form is Mitarbeiterin. Declension follows normal patterns but plural articles change. Used for coworkers; no special preposition or reflexive use either in everyday speech.
Examples
Der neue Mitarbeiter beginnt am Montag.
The new employee starts on Monday.
Unsere Mitarbeiter sind sehr freundlich.
Our staff members are very friendly.
Unsere Mitarbeiter sind sehr motiviert.
Our employees are very motivated.
Details
Mnemonics
Picture a team photo captioned 'Unsere Mitarbeiter'.
Mitarbeiter sounds like 'mit' (with) + 'arbeiter' (worker) → a worker with you: employee.
der Mitarbeiter — think masculine default for many job titles; plural often Mitarbeiter (same form).
Notes
Gendered forms exist (Mitarbeiter/Mitarbeiterin). In many contexts plural is Mitarbeiter for mixed groups.