Lost track of the rules now ...

[UntamedWisteria]

DS has invited 5 mates around this weekend.

Do they all have to stay outside (camping in garden anyway) or can they come into the house, if they stay 2 metres away from me?

It doesn't really feel like there are any rules any more!

www.gov.uk/government/publications/coronavirus-outbreak-faqs-what-you-can-and-cant-do/coronavirus-outbreak-faqs-what-you-can-and-cant-do

They can meet outdoors but have to socially distance still.

continue to meet outdoors in groups of up to six people from different households, following social distancing guidelines

stay overnight away from your home with your own household or support bubble, or with members of one other household

www.gov.uk/government/publications/coronavirus-outbreak-faqs-what-you-can-and-cant-do/coronavirus-outbreak-faqs-what-you-can-and-cant-do

The following assumes you are in England.
You are not breaking the law if you let them in the house. A gathering only becomes a criminal offence if it's over 30 people. However, if you want to follow the guidance, that says no more than two households at a time should meet indoors, and that you should still socially distance from anyone not in your own household or bubble.

To follow govt advice, they need to stay outside.