The friend system works a bit differently here than it does on some other sites, and here that isn't considered rude at all!
On SparkPeople, members can add anyone to their page to be listed as a friend. They get an email notifying them of that (unless they have disabled emails in their preferences) and can choose to add the other member as a friend or take no action at all. Sometimes when people are adding someone they haven't really had much interaction with they will leave a comment on that person's page, but there aren't really any rules or special SparkPeople etiquette you need to be aware of. I hope this helps!