I removed guests but the page still says full.

Reload the registration page; the check happens on load. If the number of guests plus companions is still at the limit, remove more or raise the limit.

Can I have a waiting list?

Not as a separate list yet. The common workaround is manual approval: leave the limit high, approve up to your real capacity and keep the rest pending as your waiting list.

Does a declined guest still take a spot?

Yes, until you remove them. Declined guests stay on the list for your records; delete them if you need the space.

Why does my registration link say "Guestlist full"?

Short answer

The event has reached its guest limit: either the capacity you set in the event settings, or the 50-guest limit of the free plan. Raise the capacity in Edit Event, remove guests who declined, or upgrade the event for unlimited guests. Companions count as guests.

Where the limit comes from

Your capacity: the guest limit you set in the event settings. When the list reaches it, the registration page shows "Guestlist full" and new sign-ups are blocked. Guests already on the list are not affected.
The free plan: free events hold up to 50 guests. The 51st registration is refused until you upgrade the event or free up spots.
Companions count: a guest with two companions uses three spots.

Fix it

Open your event, click Edit and raise the guest limit. On the free plan the maximum is 50.
Or remove guests who declined or duplicates, which frees their spots immediately.
Or upgrade: the per event upgrade (one-off) or Pro gives unlimited guests. The registration page reopens as soon as the upgrade is active.

Using the limit on purpose

A capacity is also a tool. Set it to your venue's limit and guests see "Guestlist full" instead of you having to turn people away at the door. Combine it with manual approval to keep spots for guests you want to say yes to.

Questions about this

I removed guests but the page still says full.: Reload the registration page; the check happens on load. If the number of guests plus companions is still at the limit, remove more or raise the limit.
Can I have a waiting list?: Not as a separate list yet. The common workaround is manual approval: leave the limit high, approve up to your real capacity and keep the rest pending as your waiting list.
Does a declined guest still take a spot?: Yes, until you remove them. Declined guests stay on the list for your records; delete them if you need the space.