Sharing a document library with external users

Dear all, I know this topic has been discussed a number of times and there is plenty of guidelines out there on the process.

But even so I am running into some difficulty on the best way to acheive the following scenario, which is to share a document library within a teamsite with extneral users so they can only access documents within that library and not the rest of the teamsite.

External sharing has been enabled on our teamsite so all good there.

We have a teamsite (Alpha) and within that site I created a document library (A) and broke the inheritance from the main site.

I then created a group (A-readonly) with no permissions and after that I granted that group permissions (readonly) from library A.

All good so far, but now comes the confusing part. How to give external user (someone@somewhere.com) access to the library and add them to the group I create (A-readonly).

Is this a 2 step approach e.g. sharing document/library with the user, once the user accepted (authenticated in the system) the invite I can then add them to the group or is there an easier approach.

Advice is needed on the best approach.

Cheers