Be careful when writing!

The main verb should be conjugated for Third person singular for the above indefinite pronouns even if you are thinking of more than one.

• Everyone is here at the party - correct
• Everyone are here at the party. - incorrect

The following words are occasionally used as indefinite pronouns and should be conjugated accordingly unless it is a group. a singular group is still singular.

All, any, more, most, neither, some, one

How do you know if it is a group or a collection of individuals?

Think about the idea. Are you thinking about the individuals or are you thinking about a unit?

Indefinite pronouns are formed by combining every, some, or no, with one, body, or thing.

Similiar words can be made by combining every, some, or no, with where, how, and time, however not every combination works and many are considered adverbs.

A problem arises when using possessives or object pronouns along with an indefinite pronoun. 
We use indefinite pronouns when we don't know who it is so what if it is possessive and singular (one) ?

In formal writing, English historically defaults to using him & his when unknown. If possible you can use 'one'. An alternative is to use 'his or her' or 'him or her'.

In casual writing, especially in American English, They and them are used. This technically breaks grammar since the verb no longer matches the object but they and them have no gender. It can be considered a work-around.

Another thing to consider is to use the passive structure, commonly called the passive voice.

• Someone turned on my computer -active voice
• My computer was turned on by someone. - passive voice
• Someone has lost their/his or her suitcase.
• A suitcase has been lost by someone.