Licensing requirement for accessing read only SharePoint Online team site

Thanks for all of the replies. It is interesting that it is physically possible without a licence, although I guess we have to be careful as lots of things are physically possible with Microsoft software, but it doesn't mean that it will be licensed compliantly.

It looks like you do need a license to do this, as in the OST (Online Services Terms) it mentions the below,

"Each user that accesses the Online Service must be assigned a User SL"

What we are trying to do is definitely "access the online service", therefore we will need to ensure we have a USL for these users.