There is no rule as to one mutual or few separate envelopes for family members.
You may send few applications in one and the same envelope. Filing instruction says: “If you are sending more than one case in an envelope, clearly separate the cases by rubber band or fasteners.” Just make sure that all applications are distinct. Also it is advised to attach separate checks for each application.
So you can send your family’s N-400 applications in one main envelope. However, to be safe it is advisable to include them in separate sub-envelopes marked N-400, name and A-number within the larger envelope. This will enable USCIS to identify them as separate packages. As for the terms of attending the interview and fingerprints together there is no guarantee even if the applications are filed together. You can include a cover letter requesting the same and the schedulers may accommodate, but the applications will be handled as individual applications anyway.
Meanwhile, you are not obliged to send family’s N-400 applications in the same envelope. You may use separate envelopes as well. So if you plan to send 3 or more applications, if you worry about confusion or possible delays just mail separate packages.
Also when preparing your packages, remember:
- Do not use binders or folders that cannot be easily disassembled.
- Do not use heavy-duty staples; instead use fasteners or heavy clips.
- Use fasteners to hold together thick or bulky applications. Two-hole punching the top of the material for easy placement in the file is appreciated.
- Do not submit oversized documentation unless it is necessary.