I’ve received invitations from unknown users

Receiving invitations from unknown users is rather common.
Any user can send as many invitations as they want.
There is nothing to worry about, no one can get into your network unless you accept an invitation.
You can ‘Edit your profile’ and change your private settings to hide your user name from users who don’t know its exact spelling. You can change your settings from the software: Profile > Edit my profile > Private > Check/Uncheck Hidden in approximate searches.