Entering illegally would be something like climbing the fence in some less protected spot of the land border or coming in in the boot of someones car. All other entering with passport, visa etc. is legal. Overstaying is not legal. Visitors usually get 6 months. If they wish to stay longer, they should apply to extend their visitor status before it expires. If they are applying for spousal sponsorship, especially inland, this is always allowed. With outland, almost always.
For inland sponsorship, be aware that the processing time is longer than most countries outland and you are allowed to apply outland even if your spouse is staying with you in Canada. Right now, see http://www.cic.gc.ca/english/information/times/perm-fc.asp it is 11 months for first stage approval at which time the applicant may apply for health care and is eligible for an open work permit and after that, it is another 8 months to get the PR itself. During this time, an inland applicant must be living in Canada. If the applicant leaves and for some reason can not return and this especially if they need a visa to enter Canada as they may be denied one once they are outside, they will lose their application. All that waiting time already done is poof, gone.
For outland, see here also: http://www.cic.gc.ca/english/information/times/perm-fc.asp it is 82 days in Mississauga plus go to country to check the relevant country. Those are processing times for 80% of cases so the average is somewhat faster.
You can find the forms here: http://www.cic.gc.ca/english/immigrate/sponsor/spouse-apply-how.asp#step1
The first link where it says spouse lives with you in Canada is for inland sponsorship. The second link where it says spouse lives outside Canada (although they may be visiting) is for outland.
