how to book 7-8 tickets in same sleeper train coach

Book at same time, most probably same seat mil jaati hai, nahin toh 1 ya 2 seats other coach mein aayegi, toh request karke  seat exchange kar lena

If it is a popular train where many passengers travel frequently in that route, and travel date is nearby then highly impossible. In your group if there are 45 years+ ladies and 60+ gents book them in one ticket under lower berth/Senior citizen quota (max 4 persons per ticket, if there are more than 4 try booking multiple tickets for remaining, remember all 4 should be either 45+ years females or 60+ males). If you got B3 coach for example for Senior ppl, then book for remaining persons in same B3 coach, Try booking in ladies quota for ladies irrespective of age, you can also book for male children below 12 years in Ladies quota. Remaining gents if any try to book in same B3 coach. try offline and ask clerk to book in coach of your choice or online also you may try

there is an option book only if all berth is alloted in same coach

book one ticket and then give that coach no as preferred coach in next booking

You can mention in preferences book only if all tickets are allocated in same coach

If confirm births are available then Book 4 tickets in 1 time. While booking 2nd time, enter the coach number (which u got in 1st booking) in choice and book.

Normally General quota tickets will be booked earlier than Senior citizen quota tickets.

In your case, if there is any 2 senior citizen persons out of ur tickets (Female above 45 or Male above 60), pls keep them away.

Book ticket for 5/6 non senior members in a single PNR in General quota, choosing preference Book only if all passengers aloted in same coach. 

Once the above ticket booked, Now try for remaining 1/2 high aged persons in senior citizen quota, by entering the coach from the above PNR in the preferred coach box.

Even with above combinations, still it is not sureshot that both the PNRs will be in same Coach, but it have the higher probability.

Go to the authorized railway booking counters.

They can book it for you. They will charge may be ₹10-15 extra