Alliances are a core part of the game; you can try going alone but you will be facing an uphill battle if everyone elses groups together.
Ways to form an alliance:
1. Verbal Agreement.
2. Squiring; squires are any male heir older than 12. They acquire .25 of an experience point every turn they are squiring and a full point when they are involved in a victory. They are in the control of the player they are squiring for. There is no cost to send them.
3. Fostering/lady in waiting; a child younger than 12 can be sent to another Lord to foster/lady in wait. They will not gain any experience points but are safer than a squire. Once a child becomes of age it they will become squires. Fostering does not cost anything.
4. Marriages; Marrying your daughters is a hard thing to do but it helps bring families together. There is no cost to send.
5. Non-Attack Treaties. Better known as NAT's, these allow your troops to work with another players troops and for them to pass through your occupied lands unmolested. Any player can take an army through another players land as long as there is no army there. Yes this even means your enemies. This also allows your allies nobles to enter your castle without confirmation by you.
6. Swearing fealty- you can all agree to make someone a King.
Kings: Kings are able to tax their vassals land and control their vassals troops. This is the only way for one player to control another players army.
There are two situations for oaths of fealty:
1) You have just been beaten up by someone, or been otherwise sacked. If they offer or you suggest and they accept, you can offer an oath of fealty to them and have a moderately peaceful takeover. All of your lands and incomes (except for your capital and the incomes from it) and such are effectively under their control, but your nobles will remain in your capital safe and sound(unless a player assassinates them) and you will retain control over your armies.
2) You peacefully are offered an oath of fealty before coming to arms; you are still a sovereign nation and can do as you wish but part of your income will go to the King in the form of taxes.
Trading is another core part. The ability to give another player goods and land is another factor involved with alliances.
1. Any one player can only receive a total of 4 resources from any other player or group of players. This means they can receive 4/0, 3/1, 2/2, 1/3, 0/4 total.
For example: Player 1 needs 3/1 for raising. Player 2 can send 2/1 and Player 3 can send 1/0.
2. Territories can only be given away once per turn. Turn 3 Player 1 gives Player 2 Blackwater; Blackwater can then only be returned or passed on again the next turn.