Well we need to define what is meant by a pressure group before we attempt to answer this question. Now a pressure group is a group that tries to influence public policy in the interest of a particular cause and we can see that pressure groups can aid in democracy for several reasons.
Firstly they help compensate for tyranny of the majority. This is achieved by pressure groups representing the issues of minorities and can help bring about substantive change that can help these minorities. For example the Disability Discrimination Act of 1995 came about after over 100,000 disabled people engaged in mass protest. Therefore, we can see pressure groups and pressure group activity to have helped a persecuted minority and this is undoubtedly good for democracy.
Secondly, pressure groups plug the gap in the electoral process. Without pressure groups, people may not get the chance to hold government to account apart from once every 5 years when they vote. Similarly they help compensate for election geography and by this I mean that members of a certain constituency may be voting on matters that are not confined to that constituency. So, for example, the London Cycling Campaign allows people who do not live in a London constituency the opportunity to have a say on cycling in London, through the use and membership of that said pressure group.
Finally, pressure groups encourage participation. Pressure groups can be seen to be an alternative to political parties. As political party membership has decreased, the membership of pressure groups has been on the rise. For example, the conservative party has just over 500,000 members. However, the National Trust has close to 4,000,000 members. The reason for this increase in membership of pressure groups is largely down to the significant ease of access, with sites like 38 degrees allowing for people to sign petitions online.