An alliance is a group of delegations that work on one resolution together. It forms during lobbying, it writes the resolution, and it puts the resolution to the committee.
An alliance has one main submitter and a number of co-submitters. See Main submitter and co-submitters.
Delegations that have found common ground during lobbying agree to write one text instead of several. That agreement is the alliance.
What matters is that the group is settled before drafting starts. A group that is still arguing about who is in it is not writing.
Work through this before you write:
Agree what the resolution is for. One sentence. If the group cannot write that sentence, it is not one alliance yet.
Agree who is in. Names, not intentions. A delegation that has said "maybe" is not in.
Agree who the main submitter is. Do it early and do it fairly. See Main submitter and co-submitters.
Then merge your drafts. See Merging draft resolutions.
An alliance is declared to the chairs. They then know which groups exist, who is in each one, and which resolution each group is producing. It also lets the chairs help a group that is stuck.
In MEDIMUN, all alliances are declared in MediBook and the chairs enforce this. The alliance is where the resolution is written. See Alliances in MediBook.
SCREENSHOT: /medibook/sessions/22/committees/ga1/alliances - a declared alliance with its members listed
If MediBook does not show you as a delegate. You do not count as a delegate. You cannot join the committee room. Check that your account and your seat are in order before the conference, not on day one.
If you are unsure whether your alliance has been declared. Ask a chair. An undeclared group is not a group the chairs can help, and declaring it takes a moment.
Pick delegations whose foreign policy is close to yours. That is the whole test.
Working with your own school, or with people you know, is comfortable and usually wrong. Two delegations from the same school represent different countries. Those countries often hold opposing positions on the issue.
If you are unsure whether a delegation belongs in your alliance. Ask them what their country's position is on the single hardest part of the topic. If you can both live with the answer, they belong.
If your alliance splits in two. Tell your chairs at once. Each half is a group of its own and has to be declared as one. Agree which half carries on with the draft you have written so far, before either half writes anything else.
If a delegation wants to leave. They may. A delegation cannot be forced to co-submit a text it does not support. Remove them from the alliance and check that the resolution still has the co-submitters it needs.
If the group has already started writing. Ask to join. Groups usually accept new co-submitters right up until they submit. Read the whole draft first, because you will be joining every clause in it and not only the ones you would have written.
If nobody shares your country's policy. Work with the group closest to it and use amendments during debate to move the text. A resolution you helped shape is worth more than a resolution nobody submitted. See Writing and moving an amendment.
If two delegates want to write the same clause. Let them write it separately and then merge the two. That is faster than an argument, and the merged version is usually better than either.