First of all, no parts of Scandinavia ever had feudalism in the sense that a King gave noblemen land in exchange for military service. Kingship in that sense did not exist during the viking age.

Kings were elected (or forced people to elect them) and there were no state or crown (in the sense of royal assets) to back them up. Their power were directly dependent on their personal prestige, their personal assets and the alliances with other great men they could use.

A great man, be he a Jarl, a petty King, a real King, a chieftain or simply a rich and powerful man could still not control the laws, land worked by others than his own thralls or tenants (a positon rather rare in those days) nor any military forces beyond his own hird and what he could raise from allies or friends.

There were no land registers back in those days - the first ones were created by the catholic church after Scandinavia became christian, in order to collect tithes. People knew who owned what land and the vast majority of people were self-owning peasants and free men.

The thing was the law-creating and law-enforcing authority of society, and every free man held a vote at the thing. Elders and men with good knowledge of the law were elected to pass judgement over criminals and settle disputes. Punishment ranged from fines, to exile (with your land given to the one you wronged, or his family) to death. There were no prisons.

A great man could of course swing the thing in different directions. He could often count on his relatives and the relatives of his wife. He could count on the support of those he had helped, protected or even blackmailed to support him and his proposals. Not everyone had the time and resources to come to the thing, so a great man could provide food and transport, and in return get the support of those he brought to the thing.

As for how a raid happened, I will re-post part of an old post of mine:

A Scandinavian great man decides he wants to go viking. He starts by making sure his longships are in order. During winter, his thralls burn tar and the women make new sails (a long and laborous process). The great man musters his hird, orders them to increase their training and make sure their weapon and armour is in order. He sends out messages to his allies, friends and extended family. Some are great men themselves, and have their own hirds, some are more or less professional vikings and mercenaries. All reply that they'll show up in spring.

The great man also tells his tenants and allied odalbönder of his project, and they agree to join him, providing a few more communaly owned ships to add to the great man's own.

At the winter thing, the great man will probably publically proclaim his intention, sacrifice to the Norse Gods and have wise men and soothsayers interpret the omens. Omens look good, and many of the odalbönder present at the thing promises to show up for the viking.

Come spring, the ships are fitted, the hirds mustered, stores loaded and messages sent out for those that have promised their presence. More longships show up, perhaps somene far away have heard of the expedition and decided it would be fun to join in, arrives with a set of longships and asks to join the viking.

Finally, the great man's own family, allies and hird and dependent tenants and odalbönder have mustered, say 100 hirdmen and 200 odalbönder and perhaps 50 mercenaries/professional vikings (some of which have experience of sieges and have sailed in the waters they intend to go viking in, whose presence does much to increase morale in the host).

Perhaps 100 hirdmen and 1 000 odalbönder have shown up to take part in the viking, and the fleet, now about 150 longships strong, sail off to France or England to plunder. Along the way they may attract more allies as they sail past (a great host often meant easy plunder as the King of France or any of the various Kings in England would pay ransom instead of fighting), but for the sake of this example, we'll say most others are already sailing in viking or trading.

Once they arrive, they sail up a river, guided by the experienced professional vikings and land every now and then to quickly slaughter small bands of militia and peasant levies that try to gather to counter the invading host. It takes the French (we'll assume they are French now) days to gather a levy large enough to combat the vikings, by that time, they are far off.