The deadline to be able to contribute points from the start has passed. If you want to contribute at all, you need to be in an alliance before it starts and wait until your participation timer expires before opening anything.
You need to be in an alliance for at least 14 days before you points will count towards the alliance rank rewards so you might want to find one asap.
Once they count, does the new member obtain the alliance reward tiers that were met prior to completing the minimum period?
Yes, since they would be in the alliance prior to the alliance earning those milestones.