asking someone to sit down

I know siediti mean sit down.  However if I just put please in front of it does it make it an ivitation instead of sounding like a command?

Hi David,

Siediti, per favore is more polite, but still sounds a bit informal. You could use it with a friend, with your child, wife etc. when you don't want to sound too bossy.

In formal situations, this is what you'd want to use:
Prego, si sieda pure (Please, go ahead and sit down)
This is usually accompanied by a gesture towards the empty seat. Even if it's still an order, it doesn't sound like a command, especially if you add prego and that pure at the end (translated as go ahead).
If there's more than one person, you can use:
Prego, sedetevi pure.

If you want your friend to take a seat and at the same time not sound bossy, you can use prego, siediti pure (more than one person: prego, sedetevi pure - it doesn't change from the formal form).

You could also use accomodarsi, which is a synonym of sedersi:
Basically,
sedersi = to sit down
accomodarsi = to take a seat

Prego, si accomodi pure (Please, go ahead and take a seat, formal)
Prego, accomodati pure (informal)
Prego, accomodatevi pure (pl)

I'd say accomodarsi sounds more polite than sedersi, but just because it's less common.

Hope this helps! :)

Lucia