The man seated beside the host is the guest.
Seated is an adjective meaning on a chair.
- Seating comes from the verb to seat, meaning to provide chairs to people (NOT to sit, which means to put oneself on a chair).
- Who seats also comes from the verb to seat, and would mean that the man is giving chairs near the host.
- Who seated is just the simple past of who seats, and would mean that the man was giving chairs near the host.
The glasses lying on the floor belonged to my grandmother.
Lying is an adjective that comes from the verb to lie, meaning to be flat on a surface.
- Lied is the simple past of the verb to lie, which, in this case, means to not tell the truth (this is one of the two meanings of the verb to lie).
- Lain is the past participle of the verb to lie (the correct version), but it can't be used in this sentence because it requires the auxiliary verb to have (either have lain or had lain).
- Laid is the simple past of the verb to lay, which means to put something on top of something else (this verb is commonly confused with to lie).
I understand that this may be extremely confusing. Don't worry, though, because even native English speakers get confused over the difference between to lay and to lie.
I provided an explanation here about a month ago that may help you if you are still confused.