It has a range up to 30 meters. (open field, no other signals)
The actual range in your home depends on walls, objects and other wireless signals present.

Yes you can.
Each person needs a unique account on their mobile device.
To add them as "family members" follow these steps:
In the main screen of the app tap on the name of your home (top left)
Tap on Home management
Tap on the name of your home again.
Tap on "add family members" and fill in the account details of the person you want to add.
It will give them access to all the devices in your home as a normal member or admin (you can select this)

If you only want to share a single device in a home, go to the settings menu of the device and select "share device" -> add member.

This is probably a Wifi problem, this can be for various reasons.
1.) Wifi signal is unstable.
2.) Wifi signal is too weak.
3.) Using an access point with the same SSID and password that do not support the IEEE 802.11R standards.

Problem 1 can often be solved by restarting the router.
Problem 2 can be checked with an app Wifi analyzer with this the signal can be viewed and corrected to a better position.
Problem 3 can be checked as the access points are turned off, the Wifi signal is still good and the connection remains if the access points are a problem, a Mesh solution can then be used.
Please make sure that they support IEEE 802.11R standard.