You can invite them, but they still need to apply for a tourist visa if they are in a country which normally requires one (ie, outside the EU). The type of
paperwork you need depends on your country of origin, especially whether you
are from an EEA country or not.

Without knowing more about where you are from etc I can't give very exact
advice, but in any case your best source of information is the UK Border Agency at

http://www.ukba.homeoffice.gov.uk/visas-immigration/

You should be able to find most answers you need from their website; if not,
you can contact them directly.

Generally, visa applications need to be made well in advance as there can be delays, so if your friends and family are planning a visit they should start as soon as possible. Normally there is no problem getting a visa - it just can be rather slow.