Upvote:1
I mostly do not reserve seat when booking flight with an airline having paid seat choice. When I arrive in the airport, I go to check-in counter and ask the personel for seating me where I want (not an exact point but I say the back most seats) and they mostly accept. But even if you do not do that and check-in by yourself, they mostly put the people together whose tickets bought at the same time.
Upvote:2
When you are checking in, they are typically accomodating - as much as they can.
Upvote:3
There are no guarantees. It will all depend on how full the flight is and how many other passengers pre-reserve seats.
If you want to be sure to sit together, I recommend you pay the seat reservation fee.
If you refuse to pay the fee, your best hope will be to check in as soon as allowed by your airline (I believe Lufthansa lets you check in 24 hours before your initial flight). The longer you wait, the less likely you are to sit together.
Also, your odds are better if the tickets were booked together under a single reservation. It'll be more challenging if they were booked simultaneously but separately.