Again, I used Germany for this trophy and the tactic is repeated to earn you the
Start as Germany and improve your trade market a little, you want to start getting a good GDPR. Once you've got a good bank balance rising, make sure to lower your raxes back down to avoid civil unrest and then proceed to the next stage.
Hover over a desired nation. I went with Denmark as their relationship is already pretty high. Click
on Denmark and then choose the icon for diplomacy. Select 'improve relations' with
and wait for the notice to come up that it was successful. Do it again and it should help you get over the line for what you need.
Now, clicl
over Denmark again and chose diplomacy, under here should be a bloc invitation. Hit this option and Denmark should automatically join you.
To get the afformentioned trophy above, I next targeted Norway. Well, in truth I tried Sweden and Holland first, but neither seemed to like being invited to parties, so I tried Norway. As it turns out, as Germany, you should only need to raise relations once to invite them to the bloc. Doing it a second time earns the trophy above too.