On the Free plan, 1 guest member is available for free. It's not possible to purchase extra guest members.
On all the paid plans, guests are free in a 4-to-1 ratio, which means you can have 1 free guest for every 4 registered members in your Space organization. For example, if you have 20 registered members, you can add up to 5 guest members for free.
All additional guests beyond the free allowance are billed as full members.