There's three ways to get access to the demos:
1. Buy at least the Forerunner pack ($34.95) from the Newcomer Backer Program
https://my.em8er.com/perks/campaign/5
2. If you have a $1 Stretch Goal pack (The Dollar Club) from the M3 Milestone Funding Campaign, you can upgrade it to the Deepscanner Tier ($35).
https://my.em8er.com/perks/campaign/3
Note that none of the packs listed here can be bought outright from the Backer site, someone will have to gift it to you. You can only upgrade packs to a higher tier, provided you already have any one pack from that campaign.
3. You can become a Monthly Subscriber ($7.95) to the Patreon-like program
https://my.em8er.com/perks/campaign/7
This will give you access to the builds that are one behind those available to backers (M3, Newcomer Backer). For eg. backers currently have access to the 64 player Multiplayer movement demo. The previous demo released to them was the locally hosted 5 player Multiplayer demo.
If you were to become a subscriber you would get access to the 5 player demo. Once backers receive the next build (Multiplayer movement and weapon demo), subscribers will receive access to the 64 player movement demo.
Hopefully the above makes sense.
Of all the methods, buying the Newcomer Pack is the most straightforward and quickest, as is quite obvious.