Usage of "to" and "in"

Two of the available frames for the verb "help" are:

Help [somebody] (to) [infinitive clause], eg "help you (to) finish the job"

(Note that to can usually be omitted, but it may be clearer if it is present)

and

Help [somebody] in [noun phrase] eg "help you in your work".

One particular form of noun phrase which can occur there is a gerund clause, eg "getting admission"

So, both

They are helping some candidates to get admission to the institute.

and

They are helping some candidates in getting admission to the institute.

are grammatical. The first is more natural: to me it implies that they are playing a larger part in the process - their help is crucial to the candidates' admission. In the second form, it suggests that they are giving some assistance, but the candidates could probably manage without the help.

More idiomatic in the US would be "They are helping some candidates to gain admission to the institute."