If you want to make seat reservations for 10 persons I think that you need to make the reservations by phone with the a railway company that can make the reservation.

When and where will you be travelling?

Without providing details of where and when it is impossible to give accurate advice.

For countries where reservations can be made with the operator it is sometimes possible to choose seats from seating plans.

Beware that seats are usually in bays of 4+4 and airline style 2 in 2nd class or bays of 4+2 1st class with airline 2 and 1.

If you cannot book all in one go then best to split with care, 5 and 5  or preferably 4 and 6 or even 8 and 2 depending on the group so at least all will be seated near some.

Alternative sources of reservations:

Most railways have special deptmts for what they think of as ´groups´ in practice here though these are very often very noisy schoolclasses, or other junior groups.

Most often the MAX to RES in 1 go is 5 or 6-varies again per railway